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PHOTOGRAPHY 


BY 


LIEUT.   HENRY  A.   REED,   U.  S.  ARMY, 

*> 

Assistant  Professor  of  Drawing,    U.   S.  Military  Academy,    West  Point,  N.    Y. 


NEW   YORK: 

JOHN     WILEY     &     SONS, 

15    ASTOR    PLACE. 
1888. 


COPYRIGHT,  1888, 
BY  JOHN   WILEY   &   SONS. 


DRUMMOND  &  NET, 

Electrotypers,  PRESS   OF 

1  to  7  Hague  Street, 

New  York.  FERRIS    BROTHERS. 


PREFACE. 


IT  would  seem  from  the  scarcity  of  published  information  on  the  subject  of  this 
treatise,  that  its  practical  value  has  not  been  justly  appreciated.  In  foreign  countries, 
and  particularly  in  France,  photography  has  been  for  a  considerable  period  applied  to 
surveying ;  and  the  results  show  a  saving  of  time  and  labor  that  cannot  be  attained  by 
any  other  known  means ;  and,  as  in  ordinary  surveying,  an  accuracy  that  is  directly  propor- 
tioned to  the  quality  of  the  instruments  employed  and  to  the  care  exercised  in  the  field- 
work.  In  the  United  States,  photographs  are  sometimes  used  as  addenda  to  an  important 
map,  because  they  are  recognized  as  conveying  information  to  be  otherwise  gained  only 
by  actual  inspection  of  the  tract  or  subject  represented ;  and  when  it  is  considered  that  in 
themselves  they  present  all  the  data  necessary  for  the  construction  of  a  map,  thus  rendering 
field-work  other  than  that  required  for  their  production  unnecessary,  then  certainly  this 
method  needs  but  to  be  better  known  to  be  appreciated. 

In  his  own  practice  its  value  is  so  strongly  impressed,  that  the  author  feels  it  a 
duty  to  try  and  present  the  subject  in  a  plain  and  concise  manner,  trusting  that  others 
may  so  improve  and  enlarge  upon  it,  that  this  method  of  surveying  will  not  fail  to  become 
more  generally  understood  and  practised. 

Of  the  numerous  foreign  authors  whose  recent  works  have  been  referred  to  for  in- 
formation may  be  mentioned  Laussedat,  Girard,  Gossin,  Hannot,  Bornecque,  Javary,  Tis- 
sandier,  Dufaux,  Pizzighelli,  Keucker  and  Colson.  Acknowledgment  is  also  due  for 
information  obtained  from  the  periodicals  La  Revue  cTArtillene,  La  Nature*  and  the 
Bulletin  de  la  Socie'te'  Fran^aise  de  Photographic. 

WEST  POINT,  N.  Y.,  January  23,   1888. 


CONTENTS. 


SECTION   I. 
INTRODUCTORY. — HISTORICAL  SKETCH  AND  GENERAL  PRINCIPLES,   .....  i 

SECTION   II. 
METHOD  BY  PLANE  PERSPECTIVES. — INSTRUMENTS  AND  MATERIALS,  FIELD-WORK  AND  PLOTTING,        9 

SECTION   III. 
METHOD  BY  CYLINDRIC  PERSPECTIVES. — INSTRUMENTS,  FIELD-WORK  AND  PLOTTING.      .  .  36 

SECTION    IV. 
METHOD  BY  RADIAL  PERSPECTIVES. — INSTRUMENTS,  FIELD-WORK  AND  PLOTTING,          .           .  42 

SECTION   V. 
THE  CAMERA  WITHOUT  A  LENS,     .  ........  50 

SECTION  VI. 
TELESCOPIC  AND  BALLOON  PHOTOGRAPHY,          .  .......  52 

SECTION   VII. 
VARIOUS  APPLICATIONS  OF  PHOTOGRAPHIC  SURVEYING,  AND  SOME  OF  ITS  ADVANTAGES,          .  62 


INDEX,  .  ............67 


PLATES. 
I.  PHOTOGRAPHIC  MAP,  .  .  .  .           .          .          .          .          .       End  of  Vol. 


.Cd 


PHOTOGRAPHY  APPLIED  TO    SURVEYING. 


SECTION  I. 

INTRODUCTORY. -HISTORICAL   SKETCH  AND   GENERAL   PRINCIPLES. 

HISTORICAL  SKETCH. 

I.  PERSPECTIVE  drawings  were  used  as  data  for  the  construction  of  plans  long  before 
the  practical  application  of  photography.  Near  the  beginning  of  the  present  century, 
Beautemps-Beaupr£  called  attention  to  their  use  in  topographical  surveying.  The  instruc- 
tions prepared  by  him  for  the  officers  of  the  frigate  Bonite,  preceding  a  voyage  around  the 
earth,  recommended  them  for  the  survey  of  the  surroundings  of  places  visited,  and  of  the 
coasts  which  were  either  passed  by  or  could  not  be  reached.  His  method  was  simply  to 
make  drawings  or  careful  sketches  of  the  subject  from  any  two  stations,  of  which  the 
distance  apart  was  determined ;  then,  by  measuring  with  a  sextant  or  other  means,  at 
each  station,  the  angle  included  by  a  visual  ray  to  any  point  and  the  right  line  joining 
the  stations,  data  were  afforded  for  orienting  the  sketches  on  the  plot  ;  and  a  simple 
geometrical  construction,  the  reverse  problem  of  perspective  drawing,  then  sufficed  to 
locate  the  details  in  plan.  The  accuracy  of  the  result  evidently  depended  upon  skill  in 
sketching.  Colonel  Leblanc  subsequently  endeavored  to  apply  this  method  to  military 
topography ;  but  found,  even  under  the  most  favorable  circumstances,  that  the  results 
were  only  approximately  exact ;  and,  having  but  few  advocates,  it  was  no  longer  em- 
ployed. The  result  of  these  attempts  induced  Colonel  Laussedat,  professor  of  geodesy 
at  the  polytechnic  school,  to  try  ana  find  some  means  whereby  ordinary  skill  in  draw- 
aing  would  suffice  for  the  production  of  accurate  maps;  and,  in  1854,  his  efforts  ended 
successfully  in  his  well-known  adaptation  to  the  purpose  of  Wollaston's  camera  lucida ; 
and  later  in  the  telemetrograph, — a  combination  of  the  camera  lucida  and  telescope. 

In  1839,  when  Arago  presented  Daguerre's  famous  discovery  to  the  French  Chamber 
of  Deputies,  he  said,  "  That  the  subjection  of  photographic  images  in  their  formation  to 
geometrical  rules  will  enable  us  with  very  few  data  to  determine  from  these  images  the 
exact  dimensions  of  the  highest  objects,  of  the  most  inaccessible  structures.  .  .  .  We 
could  give  some  ideas  we  have  entertained  of  rapid  means  of  investigation  that  the 
topographer  will  be  enabled  to  borrow  from  photography. 

In  1847,  Martens,  a  French  engraver,  constructed  a  panoramic  apparatus  consisting 
of  a  camera  with  a  cylindrical  daguerreotype  plate  vertical. y  disposed  ;  the  plate  remained 
stationary,  while  the  part  containing  the  objective  revolved  ;  the  light  acted  upon  the  plate 
through  a  small  vertical  slit  in  a  diaphragm  pkued  opposite  the  objective.  Sharply- 


2  PHOTOGRAPHY  APPLIED    TO   SURVEYING. 

defined  images  were  thus  produced;  but  the  preparation  of  the  .curved  plate,  and  sensitiz- 
ing it  by  the  collodion  process,  were  attended  with  too  much  difficulty  to  make  the 
apparatus  useful. 

Martens'  instrument  was  modified  by  Garella,  L.  Schuller,  and  also  by  Brandon, 
so  that  the  image  was  received  upon  a  flat  plate.  The  camera,  by  means  of  clock-work, 
was  made  to  revolve  about  a  vertical  axis  through  the  optic  centre  of  the  objective, 
carrying  with  it,  and  in  rear  of  the  objective,  a  screen  which  contained  a  vertical  slit ;  a 
narrow  sector  of  light  was  thus  admitted  ;  at  the  same  time  the  plate  was  made  to  move  and 
occupy  positions  such  as  to  be  continuously  tangent  to  the  cylinder  described  by  the 
base  of  the  sector.  Vertical  lines  were  faithfully  represented,  but  horizontals,  not  con- 
tained in  the  horizon  of  the  instrument,  were  represented  by  curves. 

Similar  modifications  were  made  by  J.  Martens  and  by  Koch;  and  Silvy  substituted 
for  the  glass  and  metal  plates  a  sensitized  paper  which  was  made  to  roll  from  one  cylin- 
der to  another. 

In  1856,  M.  Chevallier  made  his  first  researches  in  this  direction,  and  succeeded  in 
forming  upon  a  single  plate,  horizontally  disposed,  an  entire  tour  of  the  horizon.  As 
the  camera  he  employed  belongs  to  a  class  having  certain  special  merits,  a  detailed 
description  of  it  in  its  present  improved  condition  is  given  in  the  text,  par.  46.  Mangin's 
camera  also  belongs  to  this  class,  and  its  description  follows  that  of  the  former  instru- 
ment. 

In  1858,  M.  Porro,  a  maker  of  instruments  of  precision,  proposed  an  ingenious 
apparatus  intended  especially  for  topographical  purposes ;  but  the  transformation  of  the 
images  necessary  to  the  construction  of  the  plan  was  so  tedious  and  complicated,  that  it  was 
never  brought  into  use. 

Other  modifications  or  improvements  in  panoramic  cameras  were  made  in  1857  by 
Ross  and  Brooman,  in  1861  by  Sutton,  in  1864  by  Johnson,  about  1865  by  Busch,  and 
quite  recently  by  Liesegang  of  Dusseldorf ;  these  instruments  receiving  various  names, 
such  as  the  "  pantoscopic  camera,"  "  metoscopic  camera,"  and  the  "  camera  of  rotation." 

About  1858,  M.  Carette  invented  a  camera  giving  views  about  two  inches  square,  and 
insured  the  verticality  of  his  plates  by  means  of  a  circular  level.  While  he  made  the 
exposures,  an  assistant  measured  with  a  pocket  sextant  the  angles  necessary  to  fix  the 
photographic  stations.  The  negatives  were  used  directly  to  obtain  the  coordinates  of 
different  points  of  the  perspective.  This  camera  was  employed  simply  as  an  accessory 
to  the  ordinary  topographical  instruments  to  fix  some  of  the  points  upon  the  sketch. 

In  1864,  Colonel  Laussedat,  now  Director  of  the  National  Conservatory  of  Arts 
and  Trades  in  Paris,  published  in  the  Memorial  de  VOfficier  du  Gtnie  a  very  thorough 
description  of  photographic  surveying,  in  fact  so  exhaustive  that  at  the  present  date  no 
treatise  upon  this  subject  can  be  complete  without  making  use  of  the  results  of  his 
researches,  and  no  practice  perfect  without  applying  the  principles  that  he  established. 
He  may  be  called  the  inventor  of  practical  photographic  surveying.  Two  of  his  most 
extensive  undertakings  were  a  partial  survey  of  Paris  in  1861,  and  of  Grenoble  and  its 
suburbs  in  1864,  the  latter  in  charge  of  Captain  Javary.  In  each  case  the  topographical 
maps  were  characterized  by  great  accuracy ;  in  the  former,  agreeing  precisely  with  the 
regular  survey  made  by  the  engineer  in  charge  of  roads  and  bridges,  and  in  the  latter  the 


HISTORICAL   SKETCH  AND  GENERAL  PRINCIPLES.  3 

differences  of  level  as  compared  with  the  regular  survey  nowhere  exceeding  19  inches, 
which  for  ordinary  scales  is  of  course  within  the  limit  of  permissible  error. 

During  this  period,  and  while  Col.  Laussedat's  method  was  practised  in  France, 
experiments  were  made  in  Germany  by  Meydenbauer,  the  counsellor  in  charge  of  public 
buildings,  with  a  view  to  applying  photography  to  land  and  architectural  surveying. 
Apparently  unaware  of  the  researches  of  Beautemps-Beaupre  and  Laussedat,  he  first 
resorted  to  photography  for  the  construction  of  plans  and  elevations  of  certain  structures, 
difficult  of  access ;  and  like  that  of  Beautemps-Beaupre,  his  method  consisted  in  determin- 
ing the  orthographic  projection  from  the  photograph  by  reversing  the  direct  perspective 
problem.  In  land-surveying,  his  method  was  based  upon  the  same  principles  as  Lausse- 
dat's ;  but  on  account  of  the  very  narrow  field  of  view  which,  in  the  objectives  then 
employed,  corresponded  to  a  true  image,  he  would  soon  have  abandoned  his  very  costly 
experiments  had  it  not  been  for  the  optician  Busch  of  Rathenow,  who  at  this  juncture 
invented  the  pantoscope.  This  instrument,  presumably  similar  to  the  modification  of 
Martens'  heretofore  described,  gave  a  true  image  for  a  field  of  view  of  105° ;  the  only 
fault  consisting  in  an  unequal  brilliancy  of  the  centre  and  edges,  which  however  could  be 
avoided  by  securing  a  favorable  illumination  of  the  subject.  (Steinheil's  apparatus,  now  in 
use,  is  exempt  from  this  fault.)  Meydenbauer's  work  now  began  to  attract  much  atten- 
tion, and  "  photogrammetry,"  as  the  method  with  both  the  pantoscope  and  the  or- 
dinary camera  was  termed,  was  more  extensively  applied.  One  of  his  trial  surveys  was 
that  of  Fribourg  and  its  suburbs,  covering  an  area  about  half  a  mile  square,  which  was 
plotted  to  a  scale  of  ToVu>  w'tn  IO  feet  contours,  by  himself  and  a  draughtsman  who 
had  not  taken  part  in  the  field-work.  The  field-work  required  2  days ;  there  were  6  sta- 
tions and  21  plates;  the  office-work  was  performed  in  3  weeks:  and  the  resulting  map 
was  accurate  in  every  respect.  Elevations  of  buildings  were  also  obtained  with  like 
success. 

During  later  years,  several  different  instruments  for  the  purpose  have  been  invented, 
of  which  the  most  important  appear  to  be  Duboscq's  Polyconograph,  Bertsch's  Auto- 
matic Camera,  and  Dubroni's  Apparatus.  In  the  first  mentioned,  the  inner  surface  of 
the  shield  of  the  plate-holder  was  subdivided  by  raised  strips  into  fifteen  equal  squares, 
and  the  strips  being  of  such  thickness  as  to  touch  the  sensitive  plate,  the  latter  was 
practically  subdivided  in  a  like  manner.  A  metal  part  of  the  camera  containing  the 
lens  could  be  fastened  at  pleasure  opposite  any  one  of  these  squares,  and  an  exposure 
made  by  means  of  a  corresponding  opening  in  the  shield.  The  focussing  was  effected 
with  the  aid  of  a  ground  glass  placed  in  the  prolongation  of  the  sensitive  plate,  the  lens 
part  being  attached  to  it  for  this  purpose.  This  instrument  was  of  very  small  dimensions, 
the  tripods  when  folded  serving  as  a  cane. 

The  term  "  automatic"  was  applied  to  Bertsch's  camera  because  it  was  focussed  per- 
manently for  distant  views.  The  camera  consisted  of  a  four-inch  metal  cube,  a  lens  of 
four-inch  focal  distance,  and  the  sensitive  plates  were  two  and  one  half  inches  square. 
The  proper  direction  for  any  view  was  given  by  means  of  an  alidade  with  attached  level, 
having  at  one  extremity  an  open  rectangle  which,  with  the  other  extremity  of  the  alidade 
as  the  -point  of  observation,  exactly  limited  the  field.  The  negatives  produced  were  so 
sharply  defined  as  to  admit  of  a  useful  enlargement  of  from  100  to  500  times  their  super- 


4  PHOTOGRAPHY  APPLIED    TO   SURVEYING. 

ficial  area.  A  field  laboratory,  contained  in  a  box  fifteen  inches  square,  accompanied 
the  camera. 

The  main  features  of  Dubroni's  Apparatus  were  its  easy  manipulation,  and  the  use 
of  the  camera  itself  for  developing  the  plates.  The  camera  was  a  box  with  yellow  glass 
sides,  enclosed  in  another  box  of  wood  ;  openings  were  provided  as  usual  for  the  lens  and 
for  introducing  the  plate.  The  latter  having  been  collodionized  and  introduced,  the  box 
was  inverted,  lens  upward,  and  the  silver  solution  was  poured  in  through  a  tube.  The 
plate  being  thus  sensitized,  the  instrument  was  then  pointed  and  the  exposure  made ; 
the  box  was  again  inverted  and  the  plate  developed  by  introducing  the  required  solution  ; 
the  fixing  was  delayed  till  the  day's  survey  was  finished.  Since  the  focal  distance  was 
adjusted  by  the  instrument-maker,  and  rules  for  the  manipulation  accompanied  each 
instrument,  the  work  could  be  performed  by  those  not  at  all  skilled  in  photography. 

Doubtless  other  important  inventions  of  this  nature  exist  ;  but  those  mentioned  give 
an  idea  of  the  progress  made  in  the  past,  and,  together  with  the  instruments  described  in 
the  text,  will  serve  to  give  a  fair  knowledge  of  the  development  of  this  very  important 
aid  to  surveying. 

GENERAL   PRINCIPLES. 

2.  Range  and  Object  of  the  Metliod. — Although  the  range  of  Photographic  Surveying 
extends  to  geodetic  measurements,  it  is  limited  in  the  present  treatise  to  ordinary  plane 
and  topographical  work. 

As  in  surveying  with  compass  or  transit,  this  method  has  for  its  object  the  measurement 
of  angles  and  distances  necessary  to  the  location  of  points,  for  a  graphical  representation  of 
the  important  features  of  a  limited  portion  of  the  earth's  surface.  Like  an  ordinary  topo- 
graphical map  this  representation  is  a  horizontal  projection  with  added  references  for  heights ; 
but  beyond  this,  the  means  employed  furnishes,  without  additional  labor,  a  complete  view 
of  the  tract  surveyed,  with  its  details  as  they  would  be  naturally  presented  to  the  eye,  which 


D 
'-••o 


FIG.  i.  FIG.  2. 


addition  evidently  increases  the  value  of  the  result.  Its  accuracy,  as  will  hereafter  be  shown, 
may  be  said  to  be  sufficient  for  the  purposes  above  stated. 

3.  Location  of  Points. — Two  ways  of  locating  points  are  used. 

I.  By  the  intersection  of  two  or  more  right  lines  through  given  points ; — thus,  C 
(Fig.  i)  is  fixed  in  position  if  from  two  given  points,  A  and  -B,  the  directions  Aa  and 
Bb  are  known;  and  if  from  a  third  point,  D,  Dd  is  also  given,  and  Dd  intersects  C,  a 


((UNI  Vi 


HISTORICAL    SKETCH  AND    GENERAL   PRINCIPLES.  5 

check  upon  the  work  is  afforded.  It  is  evident  that  curved  lines  serve  the  same  purpose 
if  their  positions  are  known. 

C  may  also  be  fixed  if,  as  shown  in  Fig.  2,  the  distance  A' B'  and  the  angles  C'A'B' 
and  C'B'A'  are  given  ;  and,  as  before,  a  third  point  D',  being  fixed,  and  the  angle  C'D'B' 
given,  a  check  is  also  afforded. 

II.  By  an  Azimuth  and  a  Distance. — Thus,  in  Fig.  3,  the  point  E,  of  the  right  line 
EF,  given;  G  is  fixed,  if  the  azimuth  z  and  the  distance  EG  are  known.  A  check  is 
had  either  by  another  azimuth  and  distance,  or  by  the  application  of  (I),  using  other 
known  points  and  lines. 

The  foregoing  constructions  apply  to  plane-photographic  surveying.  To  locate  a 
point  completely  requires  that  not  only  shall  its  position  in  plan,  or  its  projection  upon  a 
horizontal  plane,  be  determined ;  but  also  that  its  elevation  or  depression  with  reference 
to  this  plane  shall  be  known.  Thus,  in  Fig.  4,  let  K  be  the  required  point ;  its  projection 
k  in  plan  may  be  found  by  (I)  or  (II),  e.g.,  as  indicated,  by  the  azimuth  z  and  distance 
Hk\  but  its  complete  determination  requires  the  elevation  or  distance  //above  this  plane. 
This  is  ordinarily  found  in  photographic  surveying,  by  measuring  the  angle  of  elevation 


A* 

/ 


k 
7 


/H 


FIG.  3.  FIG.  4. 

@    by   means    hereinafter   explained ;    then   h  =  Hk    tang.    @.     The    position   of    a   point 
below  the  horizontal  plane   is   similarly  determined  by  measuring  the   angle   of   depression. 

4.  Relation  of  Photographs  to  Perspective   Drawings:   Definitions  of   Terms. — The  ready 
application    of    photography   to     surveying    requires     that   the   photographs    taken    in    the 
field   shall   be    accurate   representations    of     the    features   included    in    the    field    of     view ; 
in   other   words,    that    they   shall   be    true   perspectives.     The   definitions   of   certain   terms 
used    in   perspective   drawing   may  here   be    found  appropriate.     In  Fig.  5>   the   picture   V 
represents   the    distant   topographical    features  as   shown,  and   O   the  eye   of   the   observer. 
V  is   called  the  plane  of  the  picture,  and,  for  convenience  in  the  application  of  geometric 
rules,  is   taken  in  a  vertical   position;  O   is  the  point  of  sight;   P,  the   point    in    which   a 
visual    ray   perpendicular   to  V  pierces  it,  is  called  the  principal  point ;   the   horizontal   line 
HH'  through   P  is   the  horizon  of  the  picture,  or   simply  the  horizon, — it  is   evidently  the 
intersection  with  V  of    the    observer's   horizon ;    and   the   points   in   which   visual    rays,    or 
right  lines  from    distant   points   to  O,  pierce   V,  are   the  perspectives   of  these  points;   thus 
a,  b,  .  .  .  .  are  the  perspectives  of  A,  B,  .  .  .  .      These  elements  will  be  frequently  desig- 
nated  hereafter   by  their  distinguishing  letters  as  above  given. 

5.  To   Determine   the  Direction   of  a   Point  from   a   Perspective   Representation. — If  the 
different   points   of  a   landscape   and   the  visual  rays   intersecting   them   be  vertically  pro- 


PHOTOGRAPHY  APPLIED    TO   SURVEYING 


O 


FIG.  5. 

jected   into   the   plane  of    the   horizon,    the   angles    included    by    the    projections    of    the 

visual    rays   are   the   true    horizontal   angles;    the    same    that   would   be    measured    with   a 

theodolite  or  any  other  horizontal  angle-meas- 
urer:— Thus,  in  Fig.  5,  OP  and  HH',  since 
they  are  in  this  plane,  are  their  own  projec- 
tions ;  OA  and  OB  are  horizontally  projected 
in  OA'  and  OB'  .  .  .  .;  and  a' Ob'  is  evidently 
the  horizontal  angle  included  by  OA'  and  OB'\ 
a'Oc'  the  angle  included  by  OA'  and  OC  .... 
Now  let  V,  Fig.  6,  represent  a  perspective,  as 
in  Fig.  5  ; — Since  the  positions  of  HH'  and  P, 
and  the  distance  OP,  are  always  known ;  if  V 
be  laid  upon  a  flat  surface,  PO  can  be  set  off 
perpendicularly  to  HH',  the  different  points  a, 
b,  .  .  .  .  may  be  projected  vertically  into  HH', — 
see  a',  b' ,  .  .  .  .  in  the  figure, — and  the  hori- 
zontal angle  between  any  two  points  will  then 
be  included  by  the  right  lines  joining  their 
projections  with  O.  Thus  a  Ob'  is  the  true 
horizontal  angle  of  the  points  A  and  B, — a 
and  b,  in  the  figure.  Hence  the  direction  from 

O  of   the   projection  of   any  point    on    the    horizontal    plane    may  be  determined  from  the 

perspective. 


FIG.  6. 


HISTORICAL   SKETCH  AND    GENERAL   PRINCIPLES.  7 

These  angles  may  be  measured  with  a  protractor ;  or,  since  Pa' ,  Pb'  ....  are  the 
tangents,  to  the  radius  OP,  of  the  angles  which  they  respectively  subtend,  any  angle  may  be 
determined  by  first  measuring  its  tangent  with  a  scale  of  equal  parts;  then  the  distance 
thus  obtained,  divided  by  OP,  is  the  natural  tangent  of  the  angle,  and  the  angle  is 

pa'  _j_  Pb' 

given    in    a   table  of  natural    tangents: — Thus,    tang,  a' Ob' — -^ —  .  .  .;     b'Oc'   is   ob- 
tained by  determining  POc'  and  POb'  separately,  then   b'Oc'  —  POc'  —  POV . 

The  vertical  angles  are  similarly  determined  ;  e.g.,  to  find  the  angle  of  elevation  of 
A  (a  in  the  figure) ;  set  off  a' a"  perpendicular  to  Oa!  and  equal  to  a' a,  then  measure 

a"Od  with    a   protractor ;    or,   since   a! a"  is    the    tan.    a" Oa'   to     the    radius    Oa' ,   measure 
Oa'   with   a   scale    of   equal   parts,  then   tan.    a" Oa'  —  -^— -. 

Therefore,  since  its  azimuth  and  vertical  angle  (see  also  Fig.  4)  are  given,  the 
true  direction  of  any  point  with  reference  to  O  may  be  determined  from  a  perspective 
representation. 

6.   To  Determine  the  Position  of  a  Point  from  a  Perspective  Representation. — The  other 


FIG.  7. 

element  required  to  fix  a  point  completely  is  its  distance  from  O,  or,  which  is  equivalent, 
its  direction  from  some  other  point  of  which  the  position  with  reference  to  O  is  known. 
The  latter  method  is  generally  used  in  photographic  surveying;  in  other  words,  in  this 
process,  points  are  fixed  by  intersections  from  known  points. 

This  method  requires  that  the  points  of  observation  O  and  O',  Fig.  7,  of  two  views, 
V  and  V1,  shall  be  fixed,  and  the  directions  of  OP  and  O' P  known :  if  the  positions 
of  the  horizons  HH'  are  then  given,  it  is  only  necessary  to  draw  right  lines  from  O 
and  O'  through  the  projections  of  any  point  on  these  horizons  and  produce  them  to 
intersection :— Thus,  to  fix  A  in  plan;  produce  Oa'  till  it  intersects  O'a',  and  similarly 
for  any  other  point  represented  on  both  views.  The  view  from  a  third  known  point, 


8  PHOTOGRAPHY  APPLIED    TO   SURVEYING. 

embracing  points  on  the  other  two  views,  would  serve  to  check  the  accuracy  of  the  plot. 

To  avoid  repetition,  the  orientation  of  the  views,  or  the  determination  of  the 
required  directions  of  OP  and  the  consequent  positions  of  HH'  for  the  different  views, 
is  deferred  to  Plotting,  par.  24  et  seq. 

The  remaining  element  to  determine  is  the  reference,  or  the  distance  of  any  point 
above  or  below  the  horizontal  or  datum  plane.  Referring  to  Fig.  8 ;  the  distances  Oa! 


and   a' a   being  measured  on   the  view,  and   OA'  measured  on  the   plot;    then   Oa'  :  OA' 
\\a'a  :  A' A,  whence  A' A  —  — -~—, — ;  denoting  A  A  by  H,  and   OA'  by  D,  and  since  -~— , 

is  the  tang.  @  (see  figure)  to  the  radius  Oa',  there  results  the  general  expression  for  the 

reference  of  a  point 

H=±D  tang.  @; 

the  -f-  and  —  signs  referring  respectively  to  distances  above  or  below  the  datum  plane. 

Since    the    position   of  a   point   may   thus   be    found,    any   point    may    be    completely 
determined  from  a  perspective  representation. 

The  next  step  is  to  describe  the  instruments  and  means  employed  in  the  Field-work. 


SECTION   II. 


o 


METHOD  BY  PLANE  PERSPECTIVES.-INSTRUMENTS  AND  MATERIALS,  FIELD-WORK 

AND  PLOTTING. 

INSTRUMENTS   AND   MATERIALS. 

THE  instruments  and  material  required  to  obtain  a  photographic  perspective  are  the 
camera  with  its  objective,  a  compass,  level,  certain  other  appliances  which  will  be  de- 
scribed in  their  appropriate  places,  and  the  sensitive  plates. 

7.  Camera  for  Plane  Perspectives. — Fig.  9  represents  the  kind  of  camera  used  for  this 
purpose;  it  is  the  kind  generally  used  for  photographic  work.  Its  principal  parts  are  the 
back  B,  front  F,  bellows  b, 
objective  O,  objective-slide 
D,  ground-glass  G,  rails  R, 
and  tripod  T.  H  is  the 
plate-holder,  s  its  shield, 
and  /  the  needle-points, 
of  which  the  use  is  here- 
inafter described.  Any  care- 
fully-constructed camera  of 
this  type  can  be  used  with 
good  results.  The  special 
requirements  are  that  the 
different  parts  shall  be  so 
fitted  to  each  other  as  to 
admit  of  rigid  and  uniform 
adjustment.  The  tripod 
legs  should  be  quite  stiff, 
and  when  of  the  ordinary 
telescopic  pattern,  as  in  the 
figure,  the  levelling  of  the 
instrument  is  very  quickly 
effected ;  the  binding  screw 
which  secures  the  camera 
to  the  tripod-head  should 
be  large  and  strong.  In 
focussing  with  the  special 
pattern  shown  in  the  figure, 
the  back  is  fixed  to  the  FlG 

rails,  while  the  front  is  made 

to   traverse    by   means   of    the    milled    head    shown  at  the  base  of  the    front;     but    the 
ordinary  pattern,  in  which  the  back  is  made  to  traverse,  evidently  serves  the  same  purpose. 


io  PHOTOGRAPHY  APPLIED    TO   SURVEYING. 

When  adjusted  for  taking  a  view  for  surveying  purposes,  the  optic  axis  of  the 
objective  should  be  horizontal,  and  the  film  surface  of  the  sensitive  plate — the  plane  of 
the  picture  (par.  4) — perpendicular  to  it.  This  condition  is  generally  assured  by  levelling 
the  camera  when  the  ground-glass  and  objective-slide  are  parallel ;  and  it  also  requires  that 
the  plate-holder  should  so  fit  the  back,  that  the  sensitive  plate  when  inserted  shall 
exactly  replace  the  ground-glass.  In  a  well-constructed  camera  these  conditions  either 
exist  permanently,  or  are  capable  of  adjustment  as  will  be  hereinafter  explained.  The 
form  shown  in  Fig.  9  is  called  a  "double  swing-back,"  because  the  back  can  be  turned  a 
few  degrees  about  either  a  horizontal  or  a  vertical  axis,  and  the  plate  may  thus  be  placed 
and  clamped  in  positions  oblique  to  the  optic  axis  ;  this  appliance  is  very  convenient 
for  ordinary  photographic  work,  but  unnecessary  for  the  present  purpose ;  in  fact,  with  a 
swing-back,  an  additional  adjustment  is  required  to  make  the  ground-glass  and  objective- 
slide  parallel ;  a  plain,  rectangular,  bellows  camera,  firmly  constructed,  is  the  simplest  and 
best  for  surveying  purposes.  (See  also  close  of  pars.  20  and  24.) 

8.  The  Objective. — An  objective  is  required  that  will  produce  upon  a  plane  surface  a 

true  perspective  of  a  landscape,  or  at  least  one  sufficiently  exact 
for  the  present  purpose.  There  are  doubtless  other  suitable  forms, 
but  those  known  to  the  author  are  Dallmeyer's  Rapid  Rectilinear, 
his  Wide-angle  Rectilinear,  Steinheil's  Aplanatic,  and  the  Hermagis 
90°  Angle  objectives ;  they  are  respectively  English,  German,  and 
French  patterns  adapted  to  surveying  purposes;  and  the  accuracy 
of  representation  resulting  from  their  use  is  directly  proportioned 

to  the  care    exercised  in    the   field-  and  office-work.     Each  consists 
FIG.  io. 

of  a  double  combination  of  lenses ;    each  combination  being   made 

by  joining  a  concavo-convex  lens,  «,  Fig.  TO,  of  which  the  concave  face  has  the  greater 
curvature,  and  a  meniscus,  &,  Fig.  io,  of  which  the  convex  face  has  the  greater 
curvature.  In  the  Dallmeyer  combinations,  the  former  is  of  flint,  and  the  latter  of  crown- 
glass  ;  while  in  the  Steinheil  they  are  of  flint-glass,  but  of  different  density. 

Fig.  ii  shows  the  manner  in  which  the  lenses  are  arranged  in  the  objective-tube, 
thus  forming  a  symmetrical  double  combination.  By  this  construction  and  the  use  of 
proper  diaphragms,  or  "stops,"  one  of  which,  s,  is  shown  inserted,  these  objectives  are 
corrected  for  spherical  and  chromatic  aberration  and  are  free  from  distortion;  therefore 
by  accurately  focussing,  the  images  formed  are  true  and  distinct  throughout. 

Fig.  12  represents  the  Rapid  Rectilinear  10X12,  the  greater  number  indicating  in 
inches  a  side  of  the  largest  square  plate  which  may  be  effectively  covered  by  the  image: 
the  dimensions  10X12  are  the  trade  size  of  the  plate.  The  lens-tube  T  is  attached 
by  means  of  a  screw-thread  to  the  flange  F,  which  is  fastened  by  screws  to  the  objec- 
tive-slide; the  cover  or  cap  fitting  the  extremity  E  of  the  tube,  and  used  in  making 
ordinary  exposures,  is  usually  of  leather  lined  with  velvet;  it  should  fit  closely,  and  at 
the  same  time  permit  of  removal  without  jarring  the  camera, — a  pneumatic  shutter,  ad- 
justable for  either  time  or  instantaneous  exposures,  is  much  to  be  preferred. 

9.  Compass  and  Level. — A  convenient    form    of   an    attached   compass   is  that  in  which 
the    degree    graduations   are    numbered    from   o°    to    360°,    from    right    to    left,    or   in    the 
inverse   order  of   the   numbers   on   a  watch-dial,   and   the  o°-:8o°  line   is   in   the  vertical 


JO 
V 


METHOD   BY  PLANE   PERSPECTIVES. 


plane  of  the  optic  axis  of  the  objective,  or  in  a  plane  parallel  to  it,  the  o°  being  to  the 
front;  for  in  this  case,  when  the  camera  is  pointed  toward  the  N.E.  quadrant,  the  indi- 
cations will  be  in  the  natural  order  from  o°  -to  90°,  and  so  on  throughout  a  revolution, 
thus  requiring  no  computation,  and  that  the  numbers  only  of  the  graduation  be  recorded. 


FIG.  ii. 


FIG.  12. 


FIG.   13. 


In  the  absence  of  an  attached  compass,  a  hand-compass  will  serve  the  purpose,  and  for 
convenience  while  reading  may  be  rested  upon  the  top  or  on  the  rails  of  the  camera. 

The  level  may  be  fixed  to  the  camera  by  the  instrument- 
maker  or  detached,  and  in  either  case  the  circular  form,  Fig. 
13,  is  very  convenient.  If  attached,  it  is  desirable  to  have  two 
of  this  form,  one  on  the  back  and  the  other  on  the  front  of 
the  camera:  one  is  shown  in  position  at  /,  Fig.  9. 

10.  Sensitive   Plates    and   C hanging-boxes. — During   most    of 

the  period  considered  in  par.  I,  rapid  dry  plates  were  unknown,  consequently  the  work 
was  obstructed  by  cumbersome  apparatus  and  the  delay  incident  to  long  exposures  and 
field  development.  At  the  present  time,  not  only  have  dry  plates  been  brought  to  a 
state  of  great  perfection ;  but,  where  very  light  outfits  and  great  rapidity  of  manipu- 
lation (as  instanced  in  the  roller  plate-holder)  are  elements  of  prime  importance,  paper 
has  replaced  glass.  It  is  well  to  state,  however,  in  this  connection,  that  for  exact 
work  in  surveying,  glass  negatives  have  not  yet  been  equalled;  they  retain  their  shape, 
and  the  images  are  thus  undistorted  by  chemical  manipulation ; — (see  also  par.  32.) 

Any  of  the  standard  dry  plates  are  suitable;  but  with  reference  to  detailed  repre- 
sentation, a  5X8  (inches)  is  the  smallest  that  can  be  used  to  advantage.  They  are  taken 
into  the  field  in  their  original  packing-boxes, — the  plate-holders,  of  which  six  double 
ones  are  a  convenient  number,  having  first  been  filled  in  the  dark-room.  A  negative- 
box  for  exposed  plates  is  necessary;  also  a  "field-tent,"  or  a  thick  orange  or  ruby- 
colored  cloth,  underneath  which  the  plates  may  be  transferred  as  needed  from  the 
plate-holders  to  the  negative-box,  and  from  the  packages  to  the  plate-holders.  A  chang- 
ing-box  is  also  used  for  this  purpose;  the  plates  sliding  directly  from  it  to  the  holder, 
and  the  converse,  making  the  tent  or  cloth  unnecessary. 


12  PHOTOGRAPHY  APPLIED    TO   SURVEYING. 

A  recent  invention,  due  to  Capt.  de  Torres  of  the  Spanish  army,  consists  of  a 
wooden  box  of  sufficient  dimensions  to  contain  24  plates,  which  are  held  parallel  and 
at  a  small  distance  apart  by  lateral  grooves.  One  of  its  sides,  double  the  length  of 
the  box,  slides  in  longitudinal  grooves  and  contains  a  central  vertical  slit,  large  enough 
to  permit  a  plate  to  pass  freely  through  it;  and  an  indexed  scale  and  button  on  one  of 
the  edges  serve  to  place  and  fasten  the  slit  opposite  any  plate.  A  corresponding  slit 
is  in  one  extremity  of  the  plate-holder,  and  each  slit  is  furnished  with  a  stiff  cloth 
screen  which  opens  and  closes  automatically.  To  transfer  a  plate:  the  end  .of  the  plate 
holder  is  placed  against  the  side  of  the  box,  so  that  the  slits  are  opposite  each  other; 
the  box  is  tilted,  and  the  plate  slides  into  the  holder,  where  it  rests  upon  four  small 
triangular  pieces  of  steel  placed  midway  of  the  sides  of  the  latter,  and  which  also  serve  the 
same  purpose  as  vhe  needle-points  (par.  7):  the  plate  is  held  firmly  in  place  by  a  clamp- 
screw  of  which  the  milled  head  projects  at  the  back  of  the  holder.  As  an  additional 
safeguard  against  the  admission  of  light  during  the  transfer,  the  end  of  the  holder  is 
made  to  fit  grooves  on  either  side  of  the  slit  in  the  box.  A  plate  after  exposure  is 
transferred  back  in  a  similar  manner. 

Another  and  very  simple  device  consists  in  making  the  camera-back  of  metal  and 
of  sufficient  size  to  contain,  say,  a  dozen  plates,  each  plate  contained  in  the  thin  flexible 
holder  which  is  in  common  use.  The  plates  in  place,  a  leaf-spring  attached  to  the 
rear  inner  face  of  the  box  or  back,  and  which  bends  downward,  forces  the  plates  to 
the  front.  The  cover  of  the  box  consists,  in  addition  to  the  lid,  which  is  closed  for 
transportation,  of  a  rubber  cloth,  large  enough  to  admit  of  the  shield  being  withdrawn 
within  it  from  any  holder,  and  also  to  permit  a  holder,  after  exposure  of  its  contained 
plate,  to  be  transferred  from  the  front  to  the  rear  of  the  series.  The  last  plate-holder 
of  the  series  is  readily  distinguished  by  a  notch  cut  in  its  shield. 

11.  Tests  and  Measurements  Required. — It  has  been  observed  that  for  good  work  the 
different    parts   of   a   camera    should    be    susceptible    of    rigid    and    uniform    adjustment; 
appliances  are  also  needed    in   the   field   operation   of    levelling,   to   determine  the   horizon 
of  a  view;   certain  preliminary  tests  and   measurements  are  therefore  necessary,  and  these 
taken  in  their  appropriate  order  are : 

I.  The  Test  for  Register. 
II.  The  Measurement  of  OP,  or  the  Focal  Distance. 

III.  The  Determination  of  HH'  for  any  View. 

IV.  The  Measurement  of  the  Field  of  View. 
To  these  is  added  a  Test  for  Distortion. 

Since  the  accuracy  attained  in  field-work  and  plotting  will  depend  almost  entirely 
upon  the  result  of  these  operations,  hardly  too  much  care  can  be  exercised  in  their  per- 
formance. 

12.  Test  for  Register. — When  the   plate-holder  is   inserted,  the   camera  should  "regis- 
ter;" that    is  to  say,   the    film    surface  of   the   sensitive    plate    should   exactly  replace  the 
rough  surface  of  the  ground-glass.     To  test  for  this:   focus  an  object,  a  few  yards  distant, 
accurately   on    the   latter,    using   a    large   stop;    then    substitute    for   the    sensitive  plate  a 
plate  of  ground-glass,  or  of  other  glass  covered  with  a  thin   translucent   film,  the   film   to 
the  front;    withdraw  the  shield  and  observe  if  the  image  is  as  distinct  as  before;    if  not, 


METHOD    BY  PLANE  PERSPECTIVES.  13 

note  and  mark  on  the  rails  the  distance  the  back  has  to  be  moved  to  the  front  or  rear, 
to  make  it  so,  and  adjust  accordingly  in  making  exposures.  As  in  cases  of  other  faults 
of  construction,  when  this  test  is  made  at  'first  and  the  defect  discovered,  the  instru- 
ment-maker should  remedy  it. 

13.  Measurement  of  t/te  Focal  Distance,  or  OP. — For  general  work  in  photography, 
the  focal  distance  of  an  objective,  the  distance  of  the  focus  from  the  optic  centre,  is  vari- 
able, depending  upon  the  distance  of  the  subject  from  the  optic  centre;  but,  in  survey- 
ing, since  the  nearest  points  of  the  features  to  be  represented  are  usually  so  far  distant 
that  the  rays  of  an  incident  pencil  from  any  point  may  be  regarded  as  parallel,  the 
focal  distance  for  parallel  rays  is  required.  This  for  a  combination  of  lenses  is  called 
the  equivalent  focal  distance,  but  for  the  sake  of  brevity  the  term  "  equivalent "  is  herein- 
after omitted. 

All  objectives  of  the  same  name  and  size  are  supposed  to  have  the  same  focal 
distance,  which  is  usually  stated  by  the  maker;  but  slight  variations  in  their  manu- 
facture give  different  values  for  it,  and  it  is  therefore  best  to  measure  it  for  each  one. 

For  a  simple  convex  lens,  double  or  piano  convex,   the  position  of  the  optic  centre 

S1 


s 


FIG.  14. 

is  easily  observed,  and  it  is  only  necessary  to  measure,  with  a  scale  of  equal  parts,  the 
perpendicular  distance  from  it  to  the  rough  surface  of  the  ground-glass;  but  for  a  double 
combination  lens  this  operation  is  evidently  impracticable  and  any  of  the  following 
methods  may  be  employed: 

I.  Set  up  and  level  the  camera;  place  a  staff,  S,  Fig.  14  (a  horizontal  projection), 
vertical  and  about  75  yards  from  G,  the  rough  surface  of  the  ground-glass;  and  another 
staff,  S',  also  vertical  about  10  yards  from  S,  and  so  that  55'  shall  be  perpendicular  to 
PS.  Having  drawn  a  vertical  line  through  the  middle  point  of  G,  focus  5  upon  it; 
measure  PS,— the  horizontal  distance  between  G  and  5,  —  55'  and  ss'  :  then  from  the 
similar  triangles  thus  formed,  Os  or  OP  :  ss'  ::  OS  :  55';  OP  :  ss'  ::  OP  :  ss' ;  whence 
2OP  :  2ss'  ::  OS+OP  :  SS'  +  ss';  by  substitution,  OP  :  ss'  ::  PS  :  SS' +  ss' ;  therefore 


OP  = 


PS  X  ss' 

SS'-\-ss' 


75  X  36  X  2 


e.g.:   P5=75   yards,  SS'  —  10  yards,  and  ss'  =  2  inches;    then  OP=—  —     "T~  —  !4-92 


inches,  very  nearly,  near  enough  for  practical  purposes,  ss'  can  be  measured  either  by 
exposing  a  sensitive  plate  and  applying  a  scale  of  equal  parts  to  it  after  development; 
or  on  the  rear  surface  of  the  ground-glass,  by  carefully  marking  off  on  the  edge  of  a 


I4  PHOTOGRAPHY  APPLIED    TO   SURVEYING. 

strip  of  white  paper,  the  distance  ss',  and   applying  the  scale ;    in    any   case,  ss'  and   SS' 
should  be  measured  in  the  same  direction. 

II.  Having  set  up  and   levelled   the  camera,  focus  two  points,  as  5  and   S',   Fig.    14, 
75  or   100  yards  distant,  and  so   disposed    that   their   images  on   the  ground-glass   will   be 
separated  horizontally  by  a  distance  of  about  one  third  of  its  breadth.     Make  one  image, 
as  s,  coincide  with   the  middle  vertical  of  G;   and  measure  the  distance  ss'.     Remove  the 
camera;   set  up  a  transit  or  other   angle-measurer   at   O,  and    measure   the   angle    SOS' = 
sOs',  which  denote  by  a\    then 

OP  =  — - —  =  ss'  cot.  a. 
tang,  a 

III.  It    is  a  well-known    principle  of   optics  that  when  the  sizes  of   image  and    object 
are   equal,    the    distance    between    them    is   four   times   the    equivalent    focal    distance    of 
the  lens.     Therefore,  to  measure  OP:    construct    upon  a  sheet  of  white  paper   two   right 
lines  at  right  angles  to  each  other;  from  the  point  of  intersection  as  a  centre,  and  with  a 
radius  of  about  one  third  of  the  shorter  side  of  the  ground-glass,  describe  the  circumference 
of  a  circle,  and  fasten  the  sheet  to  a  vertical  wall  or  board ;  set  up  and  level  the  camera 
in  such  a  position  that. the  image  of  the  centre  of  the  circle  shall  be  in  the  optic  axis,— 
the   middle   point   of   the   ground  glass  when    the  objective-slide    is  in  its  normal   position 
(see  par.    14);    and  focus  so  that  the  construction  on  the    paper,  and    its    image,  shall    be 
of  equal  size,  which  condition  can  be  ascertained  by  the  application  of  a  pair  of  dividers. 
Remove  the  objective  and  measure  the  distance  from  the  ground  glass  to  the  paper:  one 
fourth   of   this   distance    is   the    required   value   of    OP.      This    method    usually  requires   a 
long  camera,  or  an  extension  piece  in  front  to  hold  the  lens. 

Whenever  the  focal  distance  is  determined  by  Method  I.  or  II.,  the  positions  of  the 
parts  of  the  camera,  especially  that  of  the  back  or  front  according  to  which  one  of  these 
is  made  to  traverse,  should  be  indicated  by  marks  carefully  made  upon  the  metal  mount- 
ings, so  that  this  adjustment  can  be  readily  made  at  any  time. 

By  Method  III.,  the  position  of  the  optic  centre  can  be  indicated  upon  the 
objective-tube  by  first  replacing  the  latter,  and  then  setting  off  from  the  paper  one  half 
the  distance  between  paper  and  ground-glass,  or  2OP.  It  can  also  be  indicated  in 
Methods  I.  and  II.  by  means  which  will  suggest  themselves. 

14.  The  Determination  of  the  Horizon,  or  HH' . — First  find  the  middle  point  of  the 
ground-glass, — which  is  readily  done  by  drawing  its  two  diagonals, — and  through  it  draw 
two  right  lines  parallel  to  a  horizontal  and  a  vertical  edge  respectively  of  the  ground- 
glass  frame.  These  edges  should  be  perpendicular  to  each  other ;  but  if,  through  faulty 
construction  of  the  frame,  they  are  not  so,  then  set  up  the  camera  and  level  it,  and, 
with  the  aid  of  a  carpenter's  level,  locate  the  horizontal  line  which  passes  through  this 
middle  point,  and  draw  it ;  and  also  draw  a  vertical  line  through  the  same  point. 

The  camera  being  set  up  and  levelled,  focus  on  a  distant  point,  previously  ascer- 
tained with  a  surveyor's  level  to  be  on  a  level  with  this  horizontal  line,  and  move  the 
objective-slide  up  or  down  until  the  image  of  the  distant  point  is  cut  by  the  horizontal. 
The  objective,  or  objective-slide,  is  now  in  its  normal  position,  and  H H'  will  evidently 
pass  through  the  middle  point  of  a  view  taken  under  these  conditions. 


METHOD   BY  PLANE  PERSPECTIVES. 


It  is  apparent  that  the  amount  of  sky  or  of  foreground  in  the  view  can  be  changed 
by  raising  or  lowering  the  objective,  and  that  this  condition  usually  enables  the  surveyor 
to  adjust  his  camera  so  as  to  include  all  the  features  required,  whether  the  station  be 
elevated  or  depressed  with  reference  to  the  surrounding  features;  but  in  moving  the 
objective-slide  from  its  normal  position,  H  H'  will  no  longer  pass  through  the  middle 
point  of  the  view,  and  some  means  are  necessary  for  determining  its  true  position. 

A  simple  device  for  determining  HH'  under  these  conditions  is  as  follows:  The 
right-hand  part  of  Fig.  15  represents  a  vertical  section  through  the  optic  axis,  G  the 

K 


H 

h" 


P' 


-P 


/„ 


H' 


pro 


FIG.  15. 

ground-glass,  D  the  objective-slide ;  and  K  represents  the  plate-holder,  or,  in  case  of  a 
small  plate,  the  kit  (a  frame  fitting  within  the  plate-holder)  used  to  hold  it.  (See  also 
H,  Fig.  9.)  When  D  is  in  its  normal  position  and  an  exposure  made,  H  H'  is  marked 
upon  the  negative,  and  therefore  upon  the  print  taken  from  it,  by  means  of  the 
needle-points  /  projecting  from  the  middle  points  of  the  vertical  edges  of  the  holder 
or  kit,  and  immediately  in  front  of  the  sensitive  plate. 
The  points  p'  similarly  mark  the  position  of  the  middle 
vertical.  The  author  has  made  use  of  fine  cambric  nee- 
dles, reduced  to  about  one  third  of  their  length,  and 
heated,  before  insertion,  to  prevent  breaking  when  the  plate 
was  pressed  against  them.  For  needles,  metal  triangular 
pieces  might  be  substituted,  the  plate  resting  upon  them. 
The  true  positions  for  the  needle-points  may  evidently  be 
found  in  a  manner  similar  to  that  described  for  drawing  the 
horizontal  and  vertical  upon  the  ground-glass,  or  by  other 
simple  means.  It  is  apparent  that  when  D  is  raised  or 
lowered,  HH'  will  rise  or  fall  an  equal  distance,  and  will 
be  found  at,  say,  Jih'  or  Ji'k'" .  To  measure  this  vertical 
interval,  Hh  or  H/i",  construct  on  the  front  of  the  camera 
a  scale  of  equal  parts,  as  shown  at  c  in  Fig.  16,  and  an 
index,  which  may  be  an  edge  of  the  strap  r,  on  the  slide  will 
then  mark  the  division  denoting  the  required  interval.  The  index  should  mark  o  when 
the  slide  is  in  its  normal  position,  and  the  divisions  above  and  below  be  considered 
as  positive  and  negative  respectively.  By  this  construction  HH'  may  be  fixed  upon 
any  negative  or  print  by  setting  off  vertically  in  the  proper  direction,  from  each  of  the 
marks  produced  by  the  needle-points  /,  the  distance  measured  by  this  scale,  and  joining 
the  outer  extremities  by  a  right  line. 


FIG.  16. 


1  6  PHOTOGRAPHY  APPLIED    TO   SURVEYING. 

It  is  apparent  that  any  truly-horizontal  line  of  a  print  or  negative  may  serve  for  the 
measurement  of  horizontal  angles  ;  but  the  needle-points  usually  mark  HH'  through  the 
middle  point,  and  since  paper  prints  are  subject  to  distortion  by  chemical  manipulation 
(see  also  par.  32),  this  position  is  likely  to  give  more  exact  measurements.  (For  another 
method  of  determining  HH',  see  close  of  par.  20.) 

15.  Measurement  of  the  Field  of  View.  —  The  extent  of  the  field  of  view  of  the  camera 
is  an  important  element  in  surveying,  since  it  determines  the  amount  of  the  visible  landscape 
that  can  be  included  in  the  photograph.  It  is  measured,  horizontally,  by  the  angle  included 
by  right  lines  drawn  from  the  optic  centre  to  the  extremities  of  HH';  and,  vertically, 
by  right  lines  from  the  optic  centre  to  the  extremities  of  the  middle  vertical  of  the 
ground-glass.  The  horizontal  field  of  view  is  therefore  twice  the  angle  of  which  the 

i   H  H  ' 
tangent    is  -    np  ,    and    the   vertical    field    is    twice    the   angle    of    which     the    tangent    is 


i    middle  vertical 

~  I  e-§->  "  HH    —  9-6  inches  and  OP  •  -  16  inches,  then 


p 

i    9".  6        9 


1  6"          32 


which  practically  is  the  tangent  of   17°;    .'.  the  horizontal  field  of  view  =  34°. 

This  element  may  be  determined  graphically  by  constructing  the  angle  on  paper, 
and  then  measuring  it  with  a  protractor. 

This  is  evidently  the  field  of  view  for  the  particular  camera  to  which  the  objective  is 
attached,  and  a  like  construction  gives  it  for  any  sized  plate.  To  measure  the  field  of  view 
of  the  objective  ;  use,  instead  of  \HH',  a  radius  of  the  circle  of  light  formed  on  the  ground- 
glass,  when  the  objective  is  attached  to  a  camera  larger  than  that  to  which  it  is  suited. 

Owing  to  the  defect?,  due  to  development,  nearly  always  existing  along  the  edges  of 
a  plate,  it  is  advisable  in  the  above  measurement  to  make  an  allowance  of  2°  or  3°  for 
any  plate.  It  is  seldom  that  the  vertical  field  is  insufficient. 

When  haste  is  an  important  element,  a  wide-angle  objective,  of  which  the  focal  dis- 
tance is  much  less  and  the  field  therefore  much  wider,  may  be  used  to  great  advantage. 
In  Dallmeyer's  8xiO  rapid  rectilinear,  OP  is  13",  affording  a  field  of  about  38°;  while  in 
his  8X10  wide-angle  rectilinear,  OP  is  7",  and  the  field  is  about  54°;  therefore  with  the 
former,  10  plates  would  be  required  for  a  complete  tour  of  the  horizon  ;  but  with  the 
latter,  only  7  plates  are  needed. 

1  6.  Test  of  an  Objective  for  Distortion.  —  To  ascertain  beyond  a  doubt  that  the  objec- 
tive is  free  from  distortion  throughout  the  entire  field  of  view  assigned  to  it,  a  modifica- 
tion or  extension  of  the  method  used  for  measuring  OP  (par.  13)  may  be  employed. 
Thus,  in  Fig.  17,  a  horizontal  projection,  place  additional  vertical  staves  at  S'',  S'"  .... 
in  SS'  prolonged,  the  number  and  location  being  such  that  their  images  /',  s'"  ....  are 
well  distributed  along  any  line  of  G,  one  of  them  being  near  each  of  the  vertical  edges 
of  the  latter,  and  s  being  on  the  middle  vertical. 

Measure,  horizontally,  the  distance  from  5  to  each  staff  ;  expose  a  plate,  and,  on  the 
developed  negative,  project  s,  s'  .  .  .  .  into  HH';  from  OP  and  the  tangents  ss'",  ss"'  .... 
determine  the  angles  sOs'",  sOs{v  .  .  .  .  ;  then  compare  these  angles  with  SOS'",  SOSiv  .  .  .  , 


METHOD   BY  PLANE  PERSPECTIVES.  17 

computed  from  the  radius  OS  and  tangents  SS'",  SS{V  ....  The  corresponding  angles 
of  the  two  sets  should  be  equal.  Or  simple  proportions  may  be  used ;  thus,  ss'"  should 
be  to  SS'"  ::  Os,  or  OP,  :  OS  .  .  .  . ;  or  an  angle-measurer  may  be  substituted  as  in  measur- 
ing or. 

To  be  very   accurate,   revolve   the   objective   90°   and   repeat  the  operation. 

S" 


FIG.  17. 

Another  method  is  as  follows :  Subdivide  a  rectangle,  constructed  on  a  flat  surface, 
into  equal  squares;  make  the  dispositions  prescribed  in  III.,  par.  13,  for  paper  and  camera; 
and  compare  the  different  sides  of  the  squares  with  their  images.  When  a  value  for  OP, 
less  than  that  given  in  III.,  par.  13,  is  employed,  it  is  only  necessary  that  these  images 
should  be  proportional  reductions  of  the  original  squares.  For  convenience,  the  sides  of 
the  rectangle  should  be  proportioned  to  the  edges  of  the  ground-glass ;  and,  in  focussing, 
their  images  should  lie  near  these  edges. 

17.  Exposure,  Development  and  Printing. — Before  proceeding  to  the  Field-work,  a 
brief  description  of  the  operations  which  result  in  a  *'  view"  photograph  suited  to  survey- 
ing purposes  may  be  found  serviceable.  For  fuller  details  of  manipulation,  reference  may 
be  made  to  Manuals  of  Photography. 

(a]  The  Exposure. — The  camera  is  placed  firmly  upon  its  tripod,  levelled,  and  pointed 
toward  the  subject ;  the  back  or  front,  according  to  which  of  these  is  made  to  traverse, 
is  then  adjusted  to  the  mark  corresponding  to  the  focal  distance  (see  close  of  par.  13);  the 
objective-slide  is  so  disposed  that  the  image  will  occupy  the  most  favorable  position  upon 
the  ground-glass,  and  the  index  number  of  the  slide  is  noted.  The  proper  stop  is  then 
placed  in  the  objective ;  the  latter  is  capped  to  exclude  the  light,  and  the  plate-holder, 
containing  the  sensitive  plate,  is  inserted.  The  levelling  should  now  be  verified.  The 
shield  is  then  withdrawn,  the  cap  is  removed  for  a  period  corresponding  to  the  sensitive- 
ness of  the  plate,  and  atmospheric  and  various  other  influences  soon  observed  in  practice  ; 
the  shield  is  now  replaced.  With  a  particularly  rapid  dry  plate  (sensitiveness  25)  and  a 
bright  day,  try  the  use  of  the  smallest  stop  and  an  exposure  of  about  half  a  second.  (See 
also  par.  23.) 

(d)  The  Development. — A  printed  circular,  giving  the  process  of  development  best 
suited  to  each  of  the  many  kinds  of  dry  plates  manufactured,  usually  accompanies  each 
box  of  plates ;  but  the  following  is  a  simple  and  good  one : 


PHOTOGRAPHY  APPLIED    TO   SURVEYING. 


In  the  dark-room,  remove  the  plate  from  the  holder  and  wash  it  for  half  a  minute 
under  the  tap ;  then  develop  with  the  following  solutions  : 

No.  i, — a  filtered  sat.  sol.  of  oxalate  of  potassa  -j-  a  few  drops  of  oxalic  acid. 
No.  2, — a  filtered  sat.  sol.  of  sulphate  of  iron  --)-  a  few  drops  of  sulphuric  acid  (C.  P.). 
To  four  parts  of  No.  I  add  one  part  of  No.  2,  first  diluting  each  separately  with 
half  its  bulk  of  water ;  place  the  plate,  film  up,  in  a  shallow  porcelain  or  japanned  tray, 
and  cover  it  quickly  with  this  developer ;  keep  the  liquid  in  motion  by  gentle  rocking, 
and  develop  until  some  of  the  details  are  visible  on  the  glass  side  ;  then  wash  the  plate 
a  minute  under  the  tap,  and  place  it,  film  up,  in  the  fixing-bath,  composed  of  hyposul- 
phite of  soda  i  part  -f-  alum  T^  part  -J-  water  5  parts ;  when  clear,  that  is  when  the  white 
film  has  disappeared,  leave  it  a  few  minutes  longer  in  this  bath,  and  then  place  it  in 
running  water,  where  it  should  remain  at  least  one  hour  (five  hours  are  better).  The 
resulting  negative  is  then  permitted  to  dry  spontaneously.  A  few  drops  of  a  15-gr.  solution 
of  bromide  of  potassium  in  water,  added  to  the  developer,  will  sometimes  result  in  giving 
more  of  the  finer  details;  and  distant  mountains  may  be  brought  out  with  greater  clear- 
ness by  brushing  them  once  or  twice,  using  a  camel's-hair  brush,  with  a  5-gr.  solution  of 
this  bromide  during  the  development.  A  clearing  solution  is  sometimes  needed  after 
fixing ;  but  is  oftener  needed  with  pyro  than  with  iron  developers. 

.  (c)  The  Printing. — Albumenized  paper  is  lightly  rubbed  smooth  with  a  Canton  flannel 
pad ;  then  sensitized  by  floating  it  about  2  minutes  on  a  silver  bath  of  50  gr.  to  the  oz., — care 
being  taken  not  to  let  the  solution  touch  the  back  of  the  paper.  It  is  then,  after  thorough 
drying,  exposed  in  a  wooden  box  to  the  fumes  of  strong  ammonia,  for  from  30  to  40  min- 
utes— the  shorter  period  in  warm  weather.  The  negative  having  been  placed,  glass  face  out- 
ward, in  a  printing-frame,  Fig.  18,  a  piece  of  sensitized  paper  of  suitable  size  is  laid  carefully 

on  the  film  ;  the  back,  b,  is  pressed  down  and  secured,  and  the 
frame  is  exposed  facing  the  sun,  or  bright  sky,  until  the  lines 
of  the  print  are  slightly  bronzed,  which  condition  may  be  ob- 
served by  means  of  the  hinged  sections  of  the  back.  When 
a  sufficient  number  of  prints  have  been  made,  they  are 
washed  in  running  water  for  about  5  minutes;  then  put 
in  a  bath  of  salt  i  part  -|-  water  50  parts,  until  the  lines  turn 
red;  they  are  then  "toned"  with  chloride  of  gold,  i  gr.  to 
each  sheet  of  paper,  made  by  adding  this  to  a  solution 
of  borax  i  part  -f-  water  80  parts — a  sufficient  quantity  to  contain  the  prints ;  they  are 
left  in  this  bath  and  kept  continually  in  motion  until  the  red  lines  turn  black ;  then 
washed  a  few  minutes,  and  placed  in  the  fixing-bath,  same  as  for  plates,  and  left  10 
minutes.  They  are  then  placed  in  a  strong  solution  of  salt  and  water  for  10  minutes, 
and  finally  are  washed  for  about  5  hours  in  running  water.  After  drying,  trimming  and 
mounting,  they  become  the  ordinary  landscape  photographs. 

Other  and  simpler  processes  for  printing,  such  as  the  "  blue-process,"  using  a  negative 
to  print  from,  may  be  resorted  to;  and  in  most  cases  afford  results  sufficiently  good  for 
the  plotting. 


FIG.  18. 


METHOD   BY  PLANE  PERSPECTIVES.  19 

THE   FIELD-WORK 

1 8.  General  Considerations. — The  general  principles  of  ordinary  surveying  apply  in  the 
field-work;    the    main    conditions    to    be    fulfilled     in    order    to     ensure     satisfactory    results 
being  the  selection  of  suitable  points  of  observation,  or  stations,  and  the  proper  distribu- 
tion of  the  stations  to  obtain  favorable  intersections  and  to  economize  time. 

The  three  following  methods  of  working  are  described  : 

I.  With  the  Camera,  when  the  triangulation  has  been  established. 
II.  With  the  Camera  and  Hand-compass. 
III.  With  the  Camera  alone. 

19.  Selection  and  Distribution  of  Stations. — The  photographs  obtained  should  embrace, 
as  far  as  possible,  all  of  the  features  to  be  represented  upon  the  finished  map.     This  is  of 
easy  accomplishment  when    commanding  points    exist,  and    easier    than    might    at    first    be 
supposed  when  the  tract  to  be    represented    is    comparatively  level.      What   is  considered 
covered  ground  in  ordinary  surveying  presents  no  greater   difficulty  here ;  e.g.,  in  the  case 
of    a   heavily    wooded    section    of   the    tract,    in    working   with    transit    or   compass,    salient 
points  only  of  the  outlines  are  usually  exactly  located  from  the  commanding  stations,  the 
rest  being  filled  in   by  eye;  while  in  photographic  work,  with    usually  no  additional   labor 
in   the    field,    every  visible  curve  and   detail    of    these  outlines    may   be    plotted    from    the 
photographs  should  it  be  desirable   so  to  do. 

As  in  ordinary  surveying,  meanders  with  prismatic  compass  and  note-book  are  occa- 
sionally needed,  more  especially  to  prevent  multiplication  of  photographs  in  cases  of  views 
being  obscured  by  the  interposition  of  abrupt  salients  or  spurs;  and,  as  in  the  case  above 
given,  to  represent  roads  or  paths  through  the  covered  ground. 

The  intersection  of  lines  very  oblique  to  each  other  does  not  fix  a  point  with  clearness; 


FIG.  19. 

the  most  favorable  intersections,  in  the  case  of  two  photographs  of  the  same  section,  will  be 
afforded  when  the  photographs  are  taken  from  stations,  so  disposed  with  reference  to  each 
other,  that,  in  the  exposures,  the  optic  axes  produced  would  intersect  at  a  right  angle ; 
between  these  limits  there  is  a  wide  range  for  selection,  and  on  this  selection,  or  choice 
of  stations,  the  successful  result  greatly  depends.  For  an  ordinary  tract  of,  say,  3x4 
miles  in  extent,  three  stations  disposed,  as  shown  in  Fig.  19,  at  the  vertices  A,  B  and  C 


20 


PHOTOGRAPHY  APPLIED    TO   SURVEYING. 


of  an  approximately  equilateral  triangle,  would  doubtless  give  the  best  result.  If  the 
borders  do  not  contain  commanding  points,  three  interior  stations  similarly  disposed  might 
prove  equally  good;  but,  on  account  of  the  resulting  wider  field  at  the  different  stations, 
more  plates  would  usually  be  needed. 

An  important  element  in  this  selection,  when  exact  work  is  required,  is  that  check 
lines  shall  be  afforded  for  all  important  points;  in  other  words,  and  as  above  illustrated 
that  each  of  these  points  shall  be  found  upon  one  photograph  from  each  of  at  least  three 
stations. 

As  to  economy  of  time,   and   that  of  plates  also   may  be  added,  this  rests  principally 
the  judgment   exercised  in   the  grouping  of  the  stations.     It  is  unnecessary  that  any 
>mt   to  be  represented   should   appear  oftener  than  as  above  stated  ;  and   practice  shows 
that  the  best  arrangement  of  the  stations  is  in  groups  of  three,  each  one  of  a  group  facing 
the  exposures  at  that  station   being  confined  to,   the  same   section  of  the   tract       An 
application    of   this   principle  is  afforded  in  selecting   for  stations    the    vertices   of    triangles 
m  a  tnangulation  system;    a   complete    tour   of    the  horizon   would  then   usually  embrace 
several  triangles. 

20.  First    Method-With    the     Triangulation   Previously   Established.-^^   method    of 
development,  from  a   measured    base,  of   a  system    of   triangles  or   quadrilaterals,  of  which 
the  vertices  are  exact    positions  of   prominent    points,  is   too  well    known    to  require  a   de 
rnption    here;    ,t    is   only  necessary  that    this  system  should    be   adapted    to    the   present 
purpose,-that  the  positions  of  the  stations  should  meet  the  requirements  stated  in  par   19 
The  stations  having   been  established,  and    the  photographer  furnished  with  a   tracing 
the    triangulation    sheet  in  which    they   are    alphabetically   marked,  the    work    is   begun 
preferably  at    one  which    has   an    exceptionally    good    command  ;    for    this  may  lessen    the 
number  of   exposures  which  might    otherwise    be  required    from  one  or  more  adjacent   sta- 
ns,  without  diminishing  the  number  of  checks  required  for  good  work.       A  tour  of  the 
horizon,  or  so  much  of   it   as  may  be  needed,  is    then    made,  working   systematically  here 
as  at  all  other  stations,  from  left  to  right.       The  exposures  are  recorded  in  their  order  of 
completion    F0',    Fa',    Va\  .  .  .  ,  the   sub-letter  referring   to    the  station.       Since   the   plate 
holder  shields  are  numbered    in    the    natural    order    ,,  2,  3,  .  .  .  ;    if   the  contained    plates 
exposed  accordingly,  mistakes  will  be  obviated,  means  will  be  afforded    in  the  plotting 
as  will  be  hereinafter   shown,   for  properly   locating   the  different  views,  and  the  work  can 
be  plotted  by  those  not  present  at  the  field-operations. 

The  following-described    method  of  working   and    form  of   record  were    employed  with 
good  results  by  the  author  : 

The    camera   is  set    up,  pointing   in    the   general    direction  of   the  left-hand  view  ;    ad- 
justed to   the  focal  distance;    levelled  approximately;    then  carefully  pointed  so  as  to  in- 
lude    the  objects    on    the  extreme  left  ;    the  objective-slide   is   then  adjusted    so  that    the 
field  shall  mclude  the  important  features,  and  its  index  number  is  recorded      The 
plate-holder   is  now  inserted;    the  camera  levelled    accurately;    the  exposure  is  made,  and 
•earing  of  the  optic  axis  as  indicated    by  the  compass  is  also  recorded. 
The  camera  is  then  turned  to  the  right  for  the  next  view,  and  in  this  operation  care 
to  include  in   the  second  view  the  objects  on   the  extreme  right   of   the   field  in 
rt;    this  ,s  necessary  for  reasons  given  in  par.   15,  and  because  this  double  represen- 
.  serves  a    purpose  in    the   plotting  (see    par.  26);    a   second    exposure    is   then    made 
same   precautions  as  before  observed,  and   so  on   for  the  remaining  views 


METHOD   BY  PLANE  PERSPECTIVES. 
The    form    of    record    employed    is    here    given. 


21 


VIEW: 

No. 

Index 
Number. 

Bearing. 

A 

I 

0 

0 

2 

O 

227 

B 

I 

—    2 

84 

The  index  number    of   the  objective-slide    is    frequently  the    same    for    all  views  taken 
at  any  one  station. 

In  this  instance,  a  detached  hand-compass  was  used  ;  as  soon  as  the  plate-holder  was 
withdrawn,  the  bearing  of  a  point  of  the  image  of  any  prominent  object,  cut  by  the  mid- 
dle vertical  of  the  ground-glass,  was  taken,  and  a  description  of  the  point  was  recorded 
either  verbally  or  by  a  rough  sketch  (see  Fig.  20),  sufficiently  intelligible  to  be  recog- 
nized in  the  developed  negative.  Since  this  vertical  extended  throughout  the  field,  no 
difficulty  arose  in  finding  some  object  projected  upon  it  and  well  disposed  for  the  purpose. 
It  is  apparent  that-  the  bearing  indicated  by  an  attached  compass  immediately  after  the 
exposure,  and  before  the  camera  is 
disturbed  by  withdrawing  the  plate- 
holder,  is  likely  to  be  more  exact. 

From  a  consideration  of  the  fore- 
going, it  is  seen  that  with  suitable 
appliances  the  camera  could  be  auto- 


\\   X 

\       \ 
\ 
\ 
\ 

V 

X 

\ 

\ 

xv 

^ 

FIG.   20. 


FIG.  21. 


matically  checked  in  its  revolutions,  for  the  different  fields  in  succession ;  or,  a  hori- 
zontal limb  might  be  added  and  the  arc  of  revolution  measured  by  it,  thus  requiring 
but  one  bearing  for  a  set  of  views  from  any  station. 

A  judicious  use  of  the  hand-compass  may  frequently  serve  to  diminish  the  number 
of  views  otherwise  required.  Thus,  in  Fig.  21,  the  buildings  are  located  by  means  of  the 
view  from  station  A,  supplemented  by  the  compass  bearing  of  the  front  line  taken  from 
any  point  of  it,  as  C,  which  has  been  fixed  by  interpolation,  or  otherwise,  from  stations 


22 


PHOTOGRAPHY   APPLIED    TO    SURVEYING. 


already  determined.     This  line  evidently  intersects  the  lines  of  direction  from  A   in  required 
points  of  the  buildings  ;    similar  applications  of  the  compass  will  suggest  themselves. 

The  needle-points//',  Fig.  15,  could  be  dispensed  with  and  the  horizon,  or  HH'  of 
a  print,  be  otherwise  determined  as  follows :  At  the  station  from  which  any  view  is 
taken,  measure  with  a  hand-clinometer  the  angle  of  elevation  or  depression  of  a  promi- 
nent point,  situated  near  the  right  and  left  vertical,  respectively,  of  the  field  of  view — a 
third  point  would  serve  as  a  check,  and  its  vertical  angle  should  therefore  be  measured 
take  also  from  this  station  and  from  at  least  one  other  station  the  compass-bearings  of 
these  three  points,  so  that  their  plan  may  be  constructed.  Set  off,  in  the  proper  direc- 
tion from  the  image  of  each  point  on  the  print,  a  vertical  equal  in  length  to  the  natural 
tangent  of  the  corresponding  angle  X  distance  from  station  to  its  horizontal  pro- 
jection ;  then  the  right  line  joining  the  outer  extremities  of  the  verticals  will  be  the 
required  horizon.  It  is  apparent  that,  by  a  simple  graphical  application  of  the  "three- 
point  problem,"  the  three  compass-bearings  taken  as  here  described  might  also  serve  to 
orient  the  projected  horizons  on  the  plot. 

21.  Second  Method — With    Camera   and    Hand-compass. — There   being    no    previous  tri- 


FJG.   22. 

angulation  in  this  case,  the  stations  are  determined  by  intersecting  compass  bearings 
taken  as  the  work  progresses,  the  details  of  the  photographic  work  conforming  to  those  of 
Method  1. 

Thus,  as  illustrated  in  Fig.  22,  beginning  at  A,  a  tour  of  the  horizon,  or  so  much 
thereof  as  may  be  needed,  is  made ;  and,  if  no  natural  signal  exists  by  means  of  which  the 
camera's  position  at  A  may  be  subsequently  recognized  from  at  least  one  other  station,  as  B 


METHOD   BY  PLANE    PERSPECTIVES.  2$ 

or  C,  an  artificial  signal  is  set  up ;  this  may  be  a  white  or  black  flag  according  to  the 
nature  of  the  background  as  viewed  from  the  second  station,  whether  earth  or  sky  respec- 
tively. Proceeding  to  B,  another  tour  is  made  and  the  bearing  of  A  taken.  If  a  suit- 
able position  for  the  camera  at  C  or  D  can  be  discerned  from  B,  its  bearing  is  also 
taken  ;  and,  as  a  rule,  on  reaching  any  station,  the  bearing  of  every  other  station  to  be 
subsequently  occupied,  or  which  has  already  been  occupied,  when  its  position  can  be  discerned, 
should  be  taken  and  recorded. 

Interpolation  by  compass-bearings  is  of  much  use  here  ;  e.g.,  referring  to  the  preced- 
ing figure,  C  is  fixed  if  from  it  the  bearings  of  A  and  B  are  taken,  since  these  bearings  if 
plotted  at  A  and  B  and  produced  must  intersect  at  C. 

The  bearings  evidently  serve  to  locate  the  stations  in  their  proper  relative  positions, 
the  only  element  lacking  to  plot  to  any  required  scale  being  the  exact  length  of  some 
line  represented  in  the  view.  This  element  may  be  supplied,  as  in  ordinary  triangula- 
tion,  by  the  measurement  of  a  side  as  AB;  or  a  shorter  distance  as  ab  may  be  meas- 
ured and  the  points  A  and  B  fixed  by  intersections ;  in  either  case  the  measured  dis- 
tance is  plotted  according  to  the  desired  scale,  and  is  the  base  of  development  for  the 
plotting. 

Plotting  to  scale  may  also  be  effected  as  illustrated  in  Fig.  23.  Assume  two  points, 
as  A  and  B,  in  their  true  directions 
with  reference  to  each  other,  but  with 
the  intervening  distance  taken  at 
pleasure  ;  then,  starting  from  these 
points,  plot  the  extremities  a  and  b  of 
a  distance  which  has  been  measured 
on  the  ground  ;  also  plot  some  con- 
tained point,  as  C,  used  as  a  station. 
Construct  the  triangle  ABC;  and 
from  a  set  off  toward  b,  ab'  =  the 
measured  distance  reduced  to  the 
required  scale  ;  then  from  A  set  off 
ab'  X  AB, 


B 


FIG.  23. 


,          and  through  B'  draw  a  line   parallel  to  BC;    B'  and  C'  thus  determined 

are  the   required  positions  of  B  and   C  for  the  given  scale,  and    the  triangle  AB'C'  forms 
a  basis  of  development   for  the  plotting. 

22.  Third  Method— With  Camera  only. — In  this  case,  it  is  best  to  start  from  a  meas- 
ured base.  With  an  attached  compass,  the  bearings  of  OP  for  the  different  views  serve 
to  orient  the  views  on  the  plotting  sheet.  The  case  in  which  a  compass  is  used  for 
orienting  only,  and  that  in  which  it  is  dispensed  with  altogether,  require  no  further  de- 
scription than  that  given  for  the  plotting  in  "  Special  Cases  of  Orientation,"  par.  26.  In 
regard  to  the  stations,  careful  attention  should  be  paid  to  occupying  those  points  only 
which  may  be  clearly  recognized  in  the  views  taken  from  adjacent  stations ;  and  in  this 
respect  it  is  best  to  have  at  all  stations  visible  signals,  natural  or  artificial,  which  present 
a  strong  contrast  to  their  surroundings.  With  ordinary  care  this  method  is  quite  accurate, 
and  may  often  prove  the  only  one  available;  but,  on  account  of  the  independent  data 
afforded,  the  addition  of  compass-bearings,  of  at  least  the  stations,  is  advisable. 


24  PHOTOGRAPHY  APPLIED    TO   SURVEYING. 

23.  Best  Ti,nefor  ,he  Fi.ldvork.-f*  to  the  season  of  year:  for  wooded  tracts,  the 
best  tune  ,s  undoubtedly  the  Spring  or  Fall,  when  the  leaves  are  off  the  trees  and  the 
range  of  v,ew  is  thereby  very  much  increased  ;  the  Winter  season  also  often  affords  favorable 
opportunities.  For  tracts  not  wooded,  or  with  scattered  .trees  only,  there  is  but  little 

The  dry  plates  now  made  can  be  used  in  all  seasons  with  equally  good  effect 
As  to  the  time  of   day:   the  main    rule    to  follow  in  visiting   the  different  stations  is 
.  so  arrange  the  route,  that  at  any  station  the    camera    may  be    pointed  toward  the  sun 
as  httle  as  poss.ble.     The  most  favorable  direction  of    the  sun's  rays    is  from  the  right  or 
ft,  and  rear,  of  the   instrument;    in  some  cases,  however,  as    in   that   of   hilly   or   undu 
ung  ground,  it  is  of  advantage  to  have  them  in  a  direction    nearly  perpendicular  to  the 
c  axis,  thus  giving   the  elevations  and  depressions  greater  prominence  by  the  addition 
the,r  visible  shades  and    shadows.      In  this  respect,  a  little   observation    of    the    relative 
rees  of  clearness  with  which  various  portions  of  a  landscape    are    seen,  under   different 
Jegrees    of    inclination    of   the  sun's  rays,  will    prove    of    value.       Bouguer's    table  of    the 
tensmes  of    solar  rays  corresponding    to    different    altitudes    of    the    sun    is  very  instruc 
t,ve  here,  an.      s  given    below  ;     ,0,000  represents  the    intensity  in  a   perfectly  transparent 


SUN'S  ALTITUDE. 

INTENSITY. 

SUN'S  ALTITUDE. 

INTENSITY. 

0° 

6 

o 

2O 

5474 

I 

7 

25 

6136 

2 

192 

30 

6613 

3 

454 

40 

7237 

4 

802 

5° 

7624 

5 

1201 

70 

8016 

10 

3M9 

90 

8123 

'5 

4534 

Since  the  actm.c  effect  of  solar   rays    is  directly  proportioned    to  the  length  of  time 
during  wh,ch    they  act,  the  variations    above    given    must   be  mainly  due    to  the  d.ffeen 
d,stances  traversed  in  the  atmospheric  envelope;  for  the   same  reason,  and  because  of  the 
f  the  atmosphere,  exposures  made    in    mountainous  or  elevated  tracts  should 

lira  T:;  a: at  the  rlevel- According  to  this  tabie'  «*•-«•  -<*.  " 

.   at    8   A.M.    and   4  P.M.   should    be   of   the  same   duration ;  but,  in  practice    it  is 
found    that,   owing   to    the    comparatively  greater   amount  of   moisiure    t  the        er  1  o 

o  ;rarqduenttpartialrabsorption  °f  the  uitra-vi°iet  ** <"<  — d  4i2 

eLtof    ,'ht     "fl TH  f          'S,aIS°   ^^    ''"    la"d-P-P>«*°g~Pt>y    that    the   actinic 
light  reflected  from  wh.te  clouds  is  even  more  powerful  than  that  of  a  perfectly 
clear  sky;    brmgmg  out  the  details  of  deep  valleys  with  great  clearness 

after  a         ?£*"*   "  *"*  **"  ^^  'reqUent'y  «""«    in  Winter,  and  usually 
r  a  storm  during  all  seasons.  ' 

On   account   of   the   extreme  rap.dity  of  the  field-work  as  compared  with  that  of  any 


METHOD   BY  PLANE   PERSPECTIVES  -5 

of  the  other  methods  of  surveying,  it  is  seldom  that    the    surveyor    may  not    take    advan- 
tage of  the  most  favorable  conditions  and  circumstances. 


THE    PLOTTING. 

24.  General   Considerations. — The    plotting   of  a  survey    consists   of   the    two   following 
distinct  operations: 

I.  Constructing    the    plan,    or    the    horizontal     projections   of   the    different   points   and 
lines  ; 

II.  Determining    the    heights    of    points  with  reference    to    the   datum    plane ,   and,   as 
already  described  in  par.   6,  the    result    is    the    complete  determination  of   these  elements. 

It  seems  hardly  necessary  to  state  here  that  the  principles  defined  and  illustrated 
in  Section  I.,  and  there  confined  to  a  perspective  drawing,  apply  directly  to  a  photo- 
graphic representation.  Thus,  in  Fig.  24,  V  and  V  represent  respectively  the  relative 
positions,  with  reference  to 
the  point  of  sight,  of  a  per- 
spective drawing  and  a  pho- 
tographic representation  ;  they 
are  vertical  and  parallel,  equi- 
distant from  O,  and  the  right 
line  POP'  pierces  each  at  its 
middle  point ;  and  since 
any  point  as  D  is  projected  FIG.  24. 

by  the  same  visual  ray  in  d  and  d ',  which  are  equidistant  from  the  middle  points  of 
V  and  V ,  the  two  views  are  similar.  Another  consideration ;  since  the  maps  are  sel- 
dom constructed  to  a  scale  greater  than  ^VoiT'  an<^  because  OP  seldom  exceeds  15 
inches,  ^¥  of  an  inch  to  this  scale,  it  makes  no  sensible  difference  in  the  plotting 
whether  in  the  field-measurements  a  station-point  is  directly  beneath  the  optic  centre 
of  the  lens,  or  beneath  the  ground-glass ;  to  be  strictly  exact,  however,  it  is  apparent 
that  the  camera  should  be  so  constructed  as  to  revolve  about  a  vertical  axis  through 
the  optic  centre ;  and  that  in  setting  it  up  in  the  field  the  optic  centre  should  be 
plumbed  over  the  station-point. 

25.  To  Construct  the  Plan. — Assume  two  views,  VJ  and  Vb\  Fig.  25,  taken  from 
the  extremities  of  a  given  line,  AB,  and  embracing  the  same  objects  Having  plotted 
AB;  from  each  extremity,  with  OP  as  a  radius,  describe  circumferences  as  shown 
Draw  the  radii  A  Vu  and  BVb\  making  with  the  line  of  the  magnetic  meridian 
angles  (20°  and  280°  in  the  figure),  equal  respectively  to  the  bearings  of  OP  for  the 
views;  then  the  tangents  hti ,  at  the  extremities  of  these  radii,  are  the  required  projec 
tions  of  HH ' .  This  operation  is  termed  the  Orientation  of  the  views. 

All  the  points  which  are  to  appear  in  the  plot  having  been  projected  on  the 
respective  horizons  of  the  views  (par.  5),  and  then  transferred  by  means  of  dividers 
or  otherwise,  to  the  corresponding  projections  hh' ,  any  point  is  plotted  by  simply  draw- 
ing right  lines  from  the  plotted  stations  through  the  transferred  projections,  and  pro- 
ducing them  to  intersection  ;  thus  c,  the  intersection  of  Ac  and  Bc'\  and  d,  of  Ad'  and 


26 


PHOTOGRAPHY  APPLIED    TO   SURVEYING. 


Bd't  are  the  required  projections  of  the  actual  points   C   and  D.      If  a  view  from  a  third 
station  also  contains  these  points,  a  check  is  afforded  upon  the  accuracy  of   the   plot. 


FIG.  25. 

The  plan  of  the  entire  survey  is  plotted  in  a  similar  manner;  by  first  orienting  all 
the  views  at  their  respective  stations,  and  then  fixing  the  required  points  by  intersec- 
tions as  above. 

26.  Special    Cases    of   Orientation. — I.  Reference  has   been    made    to  the   necessity    of 

having    the     views     overlap    slightly.       This    con- 
dition  may  be    utilized,   not    only   as   a    check    in 
^    the  plotting,   but   also    to    orient   the  views  when 

^ -  compass-bearings      of     OP    have      been      omitted. 

Thus,  in  Fig.  26,  let  o'  and  o"  be  the  representa- 
tions of  the  same  object  on  the  consecutive  views 
Va  and  Fa2,  and  suppose  the  compass-bearing 
of  A  Va  is  unknown ;  to  orient  the  second  view, 
in  other  words  to  find  the  tangent-point  of  Va  : 
at  d  construct  a  tangent  to  the  given  arc,  and 
on  it  set  off  dd'  equal  to  the  horizontal  distance 
measured,  on  the  second  view,  from  the  middle  ver- 
tical to  the  image  of  O ,  join  d'  and  A  by  a  right 
line ;  then  will  V*  be  the  tangent-point  required. 

It  is  readily  seen  that  in  the  other  case,  when  the  bearing  A  V*  is  known,  the  check 
referred  to  is  afforded  by  o'  and  o"  being  on  the  same  line  of  direction. 


FIG.  26. 


METHOD   BY  PLANE  PERSPECTIVES. 


27 


II.  When  the  compass-bearing  of  some  line  of  direction  other  than  the  optic  axis 
is  given,  as  that  of  AO,  Fig.  27;  —  -graphically,  plot  at  A  the  bearing  of  O  as  indicated; 
construct  the  tangent  at  p,  and  make  pp'  equal  to  the  horizontal  distance,  measured 
on  the  print,  from  the  projection  of  o  to  the  middle  vertical  of  the  print  ;  then 
Ap'  will  intersect  the  given  arc  at  the  tangent-point  required  :  or,  trigonomctrically, 
since  the  horizontal  distance  pp'  ,  or  Vo',  is  the  tangent  of  VAo'  to  the  radius 


OP  =  A  V;    tan.  VAo'  = 


, 


and    VAo'  can    then    be    obtained    from    a  table  of    natural 


tangents  ;    therefore     first    plot    the    bearing  of  O  ;    then    protract    VAo,  and  the  side  A  V 
will  intersect  the  given   arc  at  the  tangent-point  required. 

III.  In  the  case  of  confined  space  on  the  plotting-sheet:  when  the  plotted  station  is 
near  the  border  of  the  sheet,  it  may  happen  that  the  usual  orientation  would  cause  the 
work  to  fall  outside  of  the  sheet.  This  inconvenience  can  be  avoided  either  by  propor- 
tional reduction,  or  by  reversing  the  position  of  the  transferred  horizon  ;  thus,  in  Fig.  28, 


Jiorder 


FIG.  27. 


FIG.  28. 


AV  is  the  direction  of  OP,  and  HH'  falls  without  the  border.  With  Av  =  $  or  f  . ,  .  . 
of  A  V,  as  a  radius,  describe  the  interior  concentric  arc ;  at  v,  construct  the  tangent  kh', — 
its  length  bearing  the  same  ratio  to  HH'  that  Av  does  to  AV:  then  with  sector,  pro- 
portional dividers,  or  otherwise,  transfer  the  different  horizontal  distances,  proportionally 
reduced,  from  the  view  to  hh' :  the  required  intersections  are  then  found  as  heretofore 
described. 

Or,  by  reversing  the  positions  of  the  horizons;  thus,  at  V ,  the  intersection  of  A  V 
produced  with  the  circumference,  construct  the  tangent  H" H'" .  It  is  plain  from  inspection 
of  the  figure  that  in  the  transfer,  the  horizontal  distances  are  now  to  be  set  off  from  V 

o 

in  the  reverse  direction,  e.g.,  o'  on  the  view  falling  at  o'" — the  line  of  direction  remain- 
ing unchanged.  When  necessary,  the  horizon  can  be  both  reduced  proportionally  and 
oriented  in  reverse  on  an  inner  circumference. 

IV.  Suppose   the  camera   to   have  been  used   independently  of   the  compass.     Thus, 


28 


PHOTOGRAPHY  APPLIED    TO   SURVEYING. 


in  Fig.  29,  if  the  view  from  A  contains  both  B  and  C,  and  that  from  B  both  A  and 
C:  draw  a  right  line  AB  representing  the  horizontal  distance  from  A  to  B  according  to 
the  scale  of  the  map.  With  radius  OP  describe  arcs  as  shown.  Project  the  images  a, 
b  and  c  on  the  horizons  of  their  respective  views.  To  orient  the  views:  A,  b' ,  a'  and  B 
must  be  contained  in  AB\  and  C  should  serve  as  a  check  for  accuracy.  At  t,  the  inter- 
section of  one  of  the  arcs  with  AB,  construct  the  tangent  tg,  in  length  —  the  horizontal 
distance  from  P  (par.  4)  to  6',  measured  on  the  print;  draw  Ag\  then  will  the  tangent  at 
p  be  the  projected  horizon,  /i/i',  of  the  view  from  A.  ti'ti"  is  similarly  constructed;  and 
the  subsequent  plotting  from  these  views  is  performed  as  already  described  for  other  cases. 
C  being  thus  fixed,  and  the  view  from  it  embracing  A  or  B,  the  plotting  is  extended  as 


FIG.  29. 

above  described  for  the  stations  A  and  B;  and  so  on  throughout  the  survey.  It  is  ob- 
served that  stations  to  be  occupied  must  be  so  located  as  to  be  readily  fixed  by 
intersections  from  those  already  occupied.  Numerous  opportunities  for  checking  will 
evidently  be  afforded.  (See  also  par.  22.) 

27.  To  Determine  the  Heights,  or  References. — The  general  expression  for  the  vertical 
distances  of  points  from  a  datum  plane  is  (par.  6)  H  =  ±  D  tang.  @ ;  therefore,  the 
plan  having  been  constructed,  in  order  to  obtain  the  reference  of  any  point  from  a  view, 
it  is  only  necessary  to  measure  on  the  plot  the  distance  from  the  station  to  the  point 
and  multiply  it  by  tan.  @. 

A    reference    may   be    determined    arithmetically,   trigonometrically,    or    mechanically. 

Arithmetically :    Referring  to  par.   6  and   Fig.  8,   tan.   @  =  yp ;    therefore  measure  a'a,  or 

the  vertical  distance  of  the  point  from  HH' \  divide  this  by  Oa'  and  multiply  the  quo- 
tient by  the  plotted  distance  OA' ;  the  product  is  the  required  reference.  Trigo- 
nometrically :  a'a" ,  Fig.  6,  being  equal  to  a'a  measured  on  the  view,  make  the  con- 
struction indicated  in  this  figure;  measure  a" Oa  with  a  protractor;  find  the  tan.  of 


METHOD   BY  PLANE  PERSPECTIVES. 


29 


this   angle    from    a   table    of   natural   tangents,  and    multiply  it  by  OA' .     The  mechanical 
determination  is  described  in  par.  29. 

28.  Mechanical  Measurement  of  Horizontal  and  Vertical  Angles.— To  avoid  linear  con- 
structions upon  the  negative  or  print,  the  author  has  devised  a  tangent-glass,  which  may  be 
constructed  as  follows:  select  a  flat,  clear  pane  of  glass,  a  trifle  larger  than  the  ground- 
glass  of  the  camera;  coat  one  face  with  white  wax  reduced  by  heat  to  a  thin  liquid; 
after  the  wax  is  poured  on,  it  may  be  made  to  cover  evenly  by  gently  moving  the 
glass  back  and  forth  over  a  lamp-flame.  When  cold,  rule  with  a  needle-point  two  right 
lines  HH'  and  VV,  Fig.  30,  exactly  perpendicular  to  each  other,  intersecting  at  the 


12 


14- 


h 


FIG.   30. 

middle  point  P,  and  approximately  parallel  to  the  edges  of  the  glass.  From  a  table  of 
nat.  tangents,  compute  the  tangents,  to  the  radius  OP,  first  of  £°,  then  of  i°,  i£°,  and 
so  on,  increasing  by  £°  for  each  computation,  up  to  the  number  of  degrees  required  to 
include  half  the  vertical  field  of  the  camera, — full  degrees  only  are  shown  in  the  figure. 
Set  off  these  tangents,  Pv,  Pv',  Pv*  .  .  .  .  ,  on  VV  in  each  direction  from  P;  through 
their  extremities,  v,  v',  v*  ..  .  .  ,  draw  lines  parallel  to  HH'  and  mark  them  as  shown. 
Make  a  like  construction  of  lines  parallel  to  VV.  The  ruled  face  is  then  exposed  to 
the  fumes  of  hydrofluoric  acid,  produced  by  adding  sulphuric  acid  to  fluor-spar  in  a  leaden 


30  PHOTOGRAPHY  APPLIED    TO   SURVEYING. 

dish.  The  dish  may  be  made  from  sheet-lead,  by  bending  up  the  edges  to  form  sides 
about  I  inch  in  height,  and  should  be  a  trifle  smaller  than  the  glass  so  that  the  latter 
may  rest  upon  the  edges  and  prevent  escape  of  the  fumes.  When  etched,  the  wax  is 
removed  and  the  tangent-glass  is  ready  for  use. 

To  ascertain  a  horizontal  angle,  it  is  only  necessary  to  place  the  glass  so  that  HH' 
shall  cover  the  horizon  of  the  print,  and  VV  the  middle  vertical ;  and  then  read  off 
the  required  angle.  This  position  of  the  glass  is  evidently  assured  by  means  of  the 
images  of  the  needle-points  (par.  14) ; — also  by  location  of  a  central  point  (par  20). 

With  the  usual  focal  distance,  the  sides  of  the  small  rectangles  are  of  sufficient 
length  to  admit  of  estimating  to  &  of  arc,  which  for  a  distance  of  i  mile  represents 
about  9  feet;  but  with  the  aid  of  a  microscope,  and  a  scale  of  hundredths  of  inches, 
angles  of  i'  are  easily  observed.  For  rough  work,  as  in  topographical  sketching,  the  glass 
may  be  replaced  by  tracing-cloth, — the  frame,  Fig.  34,  then  coming  in  handily. 

To  ascertain  a  vertical  angle:    Referring  to  Fig.  31,  HH'  represents  the  middle  hori- 


r\ 


\ 


H 


H' 


yi  ™    I/ 

FIG.  31. 

zontal,  and  VV  the  middle  vertical  of  the  tangent-glass,  ab  is  a  circular  arc  in  the 
plane  HOH', — this  arc  evidently  representing  the  intersection  with  this  plane,  of  a 
cylinder  of  which  the  vertical  axis  passes  through  O,  and  of  which  VV  is  the  tangent 
element  in  the  plane  of  the  picture.  From  consideration  of  the  figure,  it  is  apparent  that 
the  only  vertical  angles  which  can  be  measured  directly  with  the  glass,  on  the  view,  are 
those  of  points  contained  in  the  plane  of  O  and  VV ;  and  that  to  read  the  vertical  angles 
of  other  points,  it  is  first  necessary  to  find  their  tangents,  in  terms  of  the  radius  OP 
and  the  azimuths ;  which  may  be  effected  as  follows : 

Suppose  the  tangent-glass  laid  upon  the  view  as  described  for  the  measurement  of 
horizontal  angles,  and  o  and  /  to  be  points  of  the  view  on  the  same  horizontal.  De- 
note the  angle  pOP  by  @,  and  the  angle  oOo',  which  is  evidently  less  than  pOP,  by  @'; 
also  let  z  denote  the  azimuth  POo' .  Then  to  obtain  the  required  value  for  tan.  @': 


Pp                            OP 
tan.  @  =  -^-p ;  tan.  @    =^5-7  =  -^;    but  Oo'  = =  OP  sec.  z  ; 


PP. 

'OP 

PP 


o'o 
~Oo' 


cos.  z 


tan.  &'  — 


tan. 


OP  sec.  z     '   sec.  z 


and    since        -  =  cos.,  tan. 
sec. 


=  tan. 


cos.  2. 


METHOD   BY  PLANE  PERSPECTIVES.  31 

Therefore  on  stiff  paper  or  card-board  (or  the  glass  might  be  of  sufficient  extent  for 
the  purpose)  make  the  construction  shown  in  Fig.  32,  of  which  the  part  subdivided  into 
rectangles  is  a  duplicate  of  the  upper  half  of  Fig.  30.  From  P,  on  the  middle  vertical, 
set  off  PO  =  the  focal  distance;  draw  AB  parallel  to  HH' ;  and  produce  the  verticals 
(not  produced  in  figure)  to  meet  AB.  Let  o  be  any  point  of  the  view,  and  o"  the  intersection 
with  AB  of  the  vertical  passing  through  it :  place  a  straight-edge,  or  scale  of  equal  parts, 
on  /'and  o",  and  from  P  set  off  along  the  edge,  indicated  by  the  broken  line,  Po'"  =  o'o: 


\ 

\ 

1 

\ 
\ 

\ 

I  

\ 

\ 

\ 

\ 

^ 

\ 

\ 

1 

\ 

\ 

<0 

\ 

0 

\ 

21 

f-" 



r 

\ 

^ 

\ 

0' 

\ 

P 

H' 


00 


FIG.  32. 


the   required  vertical   angle  of  o  will  then  be  indicated  by  the  horizontal  passing  through 

o" ' .     For,  Pr  is  the  cos.  of  the  angle  o'"Pr;  and,  since  cos.  =  ,- ^- ,    the    base    or 

hypothenuse 

tan.  @'  =  tan.  @  cos.  z,  as  above.  This  scale  may  be  termed  the  vertical  angle  scale; 
and,  for  a  short  focal  distance,  may  be  conveniently  etched  upon  the  same  glass  with, 
and  as  an  extension  of,  the  tangent  scale. 

29.  Mechanical  Measurement  of  HetgJits. — The  vertical  angles  determined,  another 
scale  which  may  be  called  the  scale  of  heights  can  be  used  for  the  lineal  measurements. 
Let  CD,  Fig.  33,  represent  the  scale  of  distances  used  in  the  construction  of  the  plan. 
From  the  o  extremity  as  a  centre,  and  with  a  convenient  radius,  describe  a  circular 
arc,  which  subdivide  into  half  degrees  (whole  degrees  in  the  figure),  and  draw  radials 
as  shown.  To  measure  the  height  of  any  point,  as  O  in  the  preceding  figure:  meas- 
ure its  horizontal  distance  on  the  plot;  suppose  this  to  be  1765  feet;  at  d,  the  1765' 
division  of  the  scale,  measure  the  vertical  distance  dd'  to  the  radial  indicating  the 
vertical  angle,  in  this  case  the  3°.6o'  radial;  which  distance,  reduced  to  the  scale  of 
the  map,  is  the  height  required.  The  map  scale  itself  may  be  used  for  this  purpose, 
or  a  scale  of  equal  parts  can  be  substituted.  For  a  map  scale  of  317W  gVincn  divi- 


o" 


3* 


& 


PHOTOGRAPHY  APPLIED    TO   SURVEYING. 


sions  measure  to  within  9  feet,  and  estimation  to  half  of  this  amount  is  easily  had  by 
eye ;  but  the  application  of  y^-inch  divisions,  and  the  aid  of  a  magnifying-glass,  are 
advisable  for  more  exact  work. 


D 


FIG.  33. 

Great  exactness  may  be  obtained  by  doubling  the  length  of  the  scale  of  heights, 
and  using  correspondingly  double  the  distances  measured  on  the  plot. 

When  it  is  considered  that,  once  made,  these  scales  serve  for  all  views  taken  with 
the  same  lens,  the  slight  labor  expended  in  their  construction  is  certainly  of  little 
moment  as  compared  with  the  facility  afforded  by  their  use.  It  would  be  advantageous 
to  have  the  last  one  described  engraved  on  metal. 

30.  Reduction  of  References  to  a  Datum  Plane — The  Contouring. — In  the  determination 
of  heights  heretofore  described,  the  plane  of  reference  is  the  horizon  of  the  station  from 
which  a  view  is  taken ;  but,  as  in  all  topographical  work,  the  measurements  thus  made 
for  the  different  stations  must  all  be  reduced  to  a  single  or  common  datum  plane. 
As  previously  stated,  this  is  assumed  as  passing  through  or  below  the  lowest  point  of 
the  tract.  The  reduction  is  made  in  the  usual  manner,  by  adding  to  the  measurements 
for  any  view  the  reference  of  the  station  at  which  that  view  was  taken.  To  facili- 
tate this  operation,  a  form  of  record  similar  to  the  following  will  prove  convenient : 

FORM  OF   RECORD   FOR  REDUCTION  OF  HEIGHTS  TO   COMMON   DATUM. 


VIEW. 

POINT. 

REFERENCE. 

REFERENCE  OF  STATION. 

FINAL  REFERENCE. 

REMARKS. 

Feet. 

Feet. 

Feet. 

V1 
'  a. 

I 

-f-        102 

+      460 

+      562 

2 

+       9° 

55° 

3 

-     25 

435 

* 

* 

* 

* 

* 

No  difficulty  arises  in  determining  the  reference  in  the  fourth  column ;  each  station 
will  appear  on  at  least  one  of  the  views,  and  its  reference  is  then  obtained  the  same 
as  for  any  other  point ;  if  it  appear  on  two,  a  check  is  afforded.  In  the  case  of  re- 
ciprocal views,  the  difference  in  elevation  of  the  stations  is  readily  ascertained  from  the 


METHOD   BY  PLANE  PERSPECTIVES.  33 

relative  positions  of  the  HH'%  on  them;  and  when  the  same  point  is  represented  on 
views  from  two  stations,  the  difference  of  its  references,  as  measured  on  them,  is  evidently 
the  difference  in  height  of  the  stations. 

As  to  the  contouring,  or  plotting  of  contours:  the  reduced  or  final  references  hav- 
ing been  noted  beside  the  plotted  points,  the  views  are  examined,  and  the  confor- 
mation of  ground  carefully  observed ;  the  equidistant  points  are  then  marked  on  the 
plot,  and  the  contours  can  be  drawn  with  nearly  if  not  quite  as  much  precision  as  is 
usually  attained  in  the  "  irregular  methods"  of  contouring  with  other  instruments.  Addi- 
tional points  are  supplied  where  needed  for  this  purpose,  and  aid  is  derived  from  the 
condition,  that  all  points  cut  by  HH'  on  any  view,  have  the  same  reference  as  the 
station  from  which  that  view  is  taken.  From  the  fact  that  lines  of  direction,  from  stations 
to  points  of  profiles,  are  tangent  in  plan  to  the  corresponding  contours,  additional  aid  is 
afforded,  and  is  frequently  made  use  of  in  this  part  of  the  plotting. 

31.  Suggestions    in    Regard   to   Plotting. — The     plotting     of    a    triangulation     is    fully 
described  in  text-books   of  topographical  drawing.     The  triangulation  is  plotted  first ;  then 
come   the    orientation    of    the   views    at     plotted    stations    and    the    construction     of    the 
plan ;    and,    finally,    the    determination    of    the    heights   and     the    contouring.     It    will   be 
found    advantageous  to  fasten   together   in   their   proper   order   a  set    of   views  taken    from 
each    station,  as    the   general    view   of   the   tract   thus   afforded    gives   a   better   knowledge 
of   the    work    in    hand,    and    the    different  points  are    more  readily  recognized ;    blue-prints 
serve   this    purpose.     Each    view   should  be    marked    as   already   described,  Va\  V£     •     •    . 
V£     .     .     .    so   as   to   be   quickly   identified  ;     and,    as   soon    as    oriented,    the    transferred 
horizon    is   to   be  similarly   marked.     It    is   well    to  number,  on    the   views,   all    the   points 
to    be    plotted,    using    like    numbers   for   the    same    points    both    on    the   different    views 
and    for  their   projections    on  the   transferred    horizons.     In    the   plotting   by   intersections, 
silk   threads   may   be  used    if   desired,    instead    of    a    straight-edge :    each    thread    is    fast- 
ened   at    one    extremity   to    a    needle^   thrust    into    a    plotted    station ;    the     threads    are 
then   stretched    over   corresponding   transferred    projections   and    by   their   intersections    fix 
the   points.      The    references    of    points  should    be    marked    as   soon  as  determined,  beside 
the   plotted    points ;    and    also    recorded    for  reduction  to   a  common  datum    (par.    30) ;    for 
marking   the    final    references,    it    is    best    to    use   a   colored    pencil.      If    possible,    all    im- 
portant   points   should    be   checked    by  a   third    intersection ;    in    fact,   to  secure   a    faithful 
representation,    all    the    precautions    that  are  observed   when    other   methods   of   surveying 
are  employed,  should  be  taken. 

32.  Preparation  of  Prints — Advantage   in   Use  of  Negatives. — When    the   plotting  is  to 
be  done  from    prints,  or  regular  photographs,  these  should    be  carefully  and  equally  damp- 
ened    between    sheets   of   blotting-paper,    and    then    pasted,  without    rubbing,  to   stiff    card- 
board.    As   already   stated,    if   the    tangent-glass    is   used,    no    lines  need    be    drawn    upon 
them ;    otherwise,    the   horizons   are  drawn  ;    and    the    points    to    be    plotted,  numbered   as 
described    in     par.    31,    are    projected    by     verticals    into     their    respective     horizons.     The 
horizons  and    any   other   desired   lines   may  be    printed  from    the    negatives,    by  previously 
marking  them  on  the  latter  with  a  fine  needle-point. 

Owing  to  distortion  of  the  paper,  frequently  produced  in  the  chemical  manipulations, 
prints  are  less  reliable  than  the  glass  negatives  ;  therefore  for  exact  work  the  latter  should 


34 


PHOTOGRAPHY  APPLIED    TO   SURVEYING. 


be  used.  Sometimes,  owing  to  unfavorable  conditions  of  weather,  or  to  improperly  timed 
exposures,  negatives  do  not  show  details  with  sufficient  clearness  for  plotting  purposes.  This 
fault  can  ordinarily  be  remedied  by  using  transparencies  made  from  these  negatives,  pref- 
erably by  the  wet-plate  process,  and  which  will  usually  give  fine  definition.  Better  prints 
are  often  obtained  from  obscure  dry-plate  negatives  by  first  making  transparencies  as  above, 
and  then,  from  the  latter,  making  wet-plate  negatives  to  print  from.  If  the  plotting  is 
rapidly  done,  the  paper  prints,  if  excluded  from  light  when  not  in  use,  may  be  em- 
ployed without  subjecting  them  to  the  process  of  toning,  etc.,  and  the  consequent 
liability  to  distortion.  In  any  of  these  operations,  increased  clearness  in  definition  of  sky- 
lines, hill-profiles  and  of  other  features,  may  be  obtained,  to  almost  any  degree,  by  em- 
ploying the  usual  process  of  retouching.  Enlargements  or  reductions  to  any  desired  scale 
are  readily  effected  by  the  ordinary  photographic  process.  In  applying  the  tangent-glass, 
it  is  more  convenient  to  place  the  negative,  film  down,  upon  its  etched  face,  and  to 
rest  both  upon  a  strong  plate  of  glass  secured  to  an  open  frame,  so  disposed  that 
the  light  may  be  reflected  from  a  white  surface  upward  through  the  glass  to  the 

eye.  Fig.  34  represents  a  frame  of 
this  kind  ;  placed  in  front  of  a  window, 
and  tilted  at  any  desired  angle,  a  sheet 
of  white  paper  beneath  serves  as  a 
reflector,  and  all  but  the  reflected  light 
is  easily  excluded  by  an  opaque  curtain 
conveniently  disposed  about  the  frame 
and  draughtsman.  With  this  arrange- 
ment, the  details  are  clearly  visible, 
and  ruler  and  triangle  may  be  applied 
without  inconvenience.  The  negatives 
also  should  be  labelled  so  as  to  be  readily  identified. 

To  prepare  prints  for  accurate  plotting,  proceed  as  follows:  Construct  a  duplicate  of 
the  tangent-glass  lines  on  the  film  of  an  exposed,  blank,  developed  and  fixed  dry  plate, 
using  a  needle-point  and  marking  prominently  the  extremities  of  HH'  and  VV;  print 
these  lines  on  the  sheets  of  sensitized  paper  intended  for  the  views;  and,  in  the  sub- 
sequent printing,  be  careful  to  adjust  these  sheets  so  that  the  extremities  of  HH'  and 
VV  will  rest  exactly  upon  the  images  of  the  needle-points  on  the  station  negatives.  A 
variation  from  this  operation,  and  one  which  may  possibly  give  better  results,  is  to  expose 
each  dry  plate,  for  an  instant,  beneath  the  ruled  negative,  before  exposing  for  the  land- 
scape-views ;  the  tangent-lines  will  then  appear  in  the  latter.  Care  should  be  taken  to 
have  both  sensitive  plate  and  ruled  negative  rest  evenly  on  the  base  and  against  one 
end  of  the  plate-holder;  in  ruling  the  negative,  HH'  should  be  at  the  exact  height  of 
the  needle-points,  and  marks  made  on  the  sensitive  plate,  in  its  first  exposure,  to  show 
which  edges  of  it  to  place  up  and  against  the  end  for  the  view. 

33.  Choice  of  Scale — Accuracy  Attainable. — As  a  result  of  experience  it  has  been 
found  that  the  scale  of  j-faf,  or  of  T^,  is  the  most  convenient  for  photographic  work. 
A  smaller  scale  would  naturally  be  used  for  reconnaissances,  in  which  the  observations 
are  made  with  great  rapidity  and  therefore  as  a  rule  with  less  precision. 


FIG.  34. 


METHOD   BY  PLANE  PERSPECTIVES.  35 


As  to  the  accuracy  attainable  :  in  regard  to  the  plan  ;  admitting  an  error  of 
of  an  inch  in  the  observation  with  a  magnify  ing-glass  of  points  on  prints,  this  on 
a  circumference  of  2O-inch  radius  (corresponding  to  a  focal  distance  of  20  inches) 
represents  an  angle  of  about  45",  and  the  maximum  error  in  the  location  of  a  point 
2700  yards  distant  would  therefore  be  but  20  inches,  which,  reduced  to  the  scale  of 
would  of  course  be  inappreciable.  For  the  heights;  assuming  a  point  1000  yards 


/     OO/l 

distant    and    the    same    focal    distance,    the    error    would    be    --  '—  -  =  7.2    inches, 

20 

which  corresponds  to  an  angular  error  of  about  30",  and,  in  observing  with  ordinary 
instruments,  this  degree  of  accuracy  would  seldom  be  excelled. 

The  facts  set  forth  in  this  paragraph  are  results  of  the  observations  made  at  a 
time  when  photographic  instruments  were  in  a  less  perfect  condition  than  at  present. 
In  the  author's  experience  during  the  past  year,  in  the  outline  photographic  sur- 
vey of  a  tract  of  about  12  square  miles,  using  the  camera  represented  in  Fig.  9,  a  Dall- 
meyer  rapid  rectilinear  objective,  with  OP  —  15  ".68,  and  plotting  from  the  negatives 
with  the  aid  oi  the  tangent  glass,  points  of  the  plan  checked  exactly  and  in  plotting 
the  levelling,  the  difference  in  heights  of  points  at  various  distances  corresponded  to 
the  measurements  made  by  careful  work  with  a  surveyor's  level.  The  scale  of  the  plot 
in  this  case  was  ro^To  and  the  field-work  for  the  entire  tract  required  but  half  a  day. 

It    may   be   said    that    the   accuracy  attainable    is    sufficient  for   all    ordinary  purposes. 

34.  Explanation  of  Plate  /,  —  a  Photographic  Map  (see  end  of  book],  —  Plate  I  (Plate 
XXIV,  in  "Topographical  Drawing  and  Sketching"),  from  the  Memorial  de  l'Officier  du 
Genie,  represents  a  photographic  map  of  an  area  of  4500  hectares,  —  in  English  units 
the  rectangle  is  about  3.1  X  4-4  miles.  The  field-work  was  first  plotted  to  the  scale 
of  s-ffVu"'  and  afterwards  reduced  by  photo-lithography  to  its  present  scale,  -g-g-J-oTr-  The 
small  triangles,  A,  mark  the  vertices  of  the  triangulation  ;  these  and  the  other  points 
marked  by  a  small  circle,  O,  are  the  photographic  stations,  —  the  minor  triangulation, 
or  traversing,  by  which  the  Qs  were  fixed,  is  omitted.  The  base  was  measured  in 
the  valley  E.  of  Sainte-Marie-aux-Mines,  and  served,  as  shown,  to  fix  points  I  and 
2,  and  the  directions  of  points  4,  5,  16  and  17:  the  side  1-2  was  then  used  to 
fix  the  points  3,  4,  10,  .....  The  contours  have  an  equidistance  of  5  metres  ; 
the  field-work  occupied  10  days;  the  number  of  photographic  stations  was  31  ;  of 
prints,  52  ;  and  of  points  determined,  1400.  This  survey  was  conducted  by  M. 
Javary  in  1869. 


SECTION  III. 


CYLINDRIC  PERSPECTIVES.— INSTRUMENTS,  FIELD-WORK  AND   PLOTTING. 

35.  The      Topographic     Cylindrograph. — This     ingenious    instrument,    a    type    of    the 
cylindro-perspective   class   of   cameras,    is   the    recent    invention    of    M.    Moessard,  formerly 
professor    of    topography    at     St.    Cyr,    now    commandant    of     engineers     and     professor 
"a   1'Ecole    Superieure   de    Guerre." 

Views  obtained  with  it  are  cylindric  perspectives;  the  radius, — the  focal  distance, — 
is  a  constant  quantity  for  vertical  as  well  as  horizontal  angles ;  and  the  number  of 
views  required  for  a  complete  tour  of  the  horizon  is  but  two  and  a  fraction  (20°). 
Fig-  35  is  a  general  view  of  the  apparatus. 

36.  The   Camera. — The    camera    consists    of     a     semicircular   wooden    lid   A,    Fig.    36, 
and    base   £,  which  are    horizontally  disposed :    and    a    vertical,  rectangular,  wooden    frame 

F.  These  pieces  are  fastened  together  with  hinges 
and  hooks  as  shown,  so  that  they  may  be  readily 
folded  for  transportation.  When  opened,  a  brass 
standard  s,  attached  to  the  lid  in  rear,  rests  upon 
the  base,  where  it  is  fastened  in  position  by  a 
button,  and  thus  serves  to  keep  these  parts  rigidly 
parallel ;  this  may  also  be  folded  against  the  lid 


FIG.  35. 


FIG.  36. 


FIG.  37. 


for  transportation.  Strips  b,  of  brass,  project  vertically  0.4  of  an  inch  beyond  the  edges. 
The  projections,  e,  Fig.  37,  on  the  upper  surface  of  the  base,  are  guards  or  guides  for 
the  flexible  plate-holder  (see  Fig.  40),  which,  when  in  position  for  an  exposure,  forms  the 
cylindrical  part  of  the  camera.  Projecting  pieces,  /  and  c,  in  rear  of  the  base,  serve  as  sup- 
ports for  the  circular  level,  and  for  the  compass,  which  is  graduated  from  o°  to  360°  in  the 
usual  direction,  N.,  E.,  S.  and  W.,  and  which  has  its  o°-i8o°  line  parallel  to  the  middle 
radius  of  the  lid,  with  the  o°  point  to  the  front.  In  the  latest  form  of  the  instrument, 
two  tube  levels  at  right  angles,  and  the  compass,  4.7  inches  in  diameter,  are  let  into  the 


C  YLIND  RIC  PERSPECTI VES. 


37 


upper  surface  of   the  lid.     A  plate  in  the  lower  surface    of   the    base    serves   to  attach  the 
camera  to  the  tripod-head. 

37.  The  Objective. — The  objective-carrier,  shown  in  Fig.  38,  is  so  attached  by  a 
vertical  spindle  to  the  middle  of  the  frame  F  (Fig.  36),  that  the  optic  axis  of  the  objec- 
tive is  horizontal  and  the  optic  centre,  or,  in  the  case  of  a  single  lens,  the  rear  nodal 
point,  is  in  the  axis  of  the  camera.  Fig.  38  shows  the  arrangement ;  the  objective  o  is 
rigidly  fastened  to  the  rear  plate  /,  and  enters  freely  an  opening  in  the  front  plate  /, 
the  plates  being  held  together  by  adjusting  screws  and  spiral  springs  as  shown.  Fig.  35 
shows  the  manner  in  which  the  objective  may  be  made  to  revolve  horizontally, — the 
alidade,  attached  to  the  upper  rectangular  extremity  of  the  spindle  /  (Fig.  38),  serving  as 
a  handle.  The  other  extremity  of  the  alidade  traverses  a  circular  scale  of  degrees,  which  is 
attached  to  or  engraved  upon  the  lid,  and  which  has  its  centre  in  the  axis  of  rotation ;  the 
alidade  is  also  furnished  with  a  spring  of  slight  tension,  a  click,  projecting  beneath  and 
engaging  in  notches  (Fig.  35)  cut  in  the  lid  ;  which  device  serves  to  regulate  the  rate  of 
motion  of  the  alidade,  by  assigning  so  many  clicks  per  second,  and  thereby  to  time 
the  exposure.  In  front,  light  is  excluded  from  the  interior  of  the  camera  by  a  rubber 
cloth,  attached  to  the  rear  plate  of  the  objective  and  the  inner  edges  of  the  vertical 
frame,  and  of  such  dimensions  as  to  permit  perfect  freedom  of  motion  to  the  objective. 


FIG.  39. 


Fiu  40. 


To  confine  the  deviated  rays  to  the  area  effectively  covered  by  the  objective,  there 
are  two  wings  or  screens,  w,  Fig.  39,  of  tin  blackened,  attached  to  the  rear  plate  of  the 
objective-carrier  and  projecting  into  the  camera;  each  turns  freely  about  a  vertical  axis 
a,  and,  by  means  of  the  notches  n  in  the  rear  projecting  piece,  may  be  made  to  occupy 
three  different  positions,  thus  increasing  or  diminishing  the  breadth  of  field  :  the  pro- 
jecting piece  and  screens  may  be  folded  against  the  plate  for  transportation.  The 
screens  should  be  of  such  size  as  to  turn  freely  in  the  camera  during  the  rotation  of 
the  objective.  The  folding  vanes  (Fig.  35)  with  cross-hairs,  attached  to  the  alidade,  serve 
to  determine  at  any  instant  that  portion  of  the  landscape  which  is  being  "  taken "  when 
the  screens  are  parallel. 

Any  rectilinear  objective,  composed  of  either  a  single  or  double  combination  lens,  will 
serve,  provided  its  focal  distance  is  equal  to  or  a  very  little  less  than  the  radius  of  the 
camera. 

38.  The  Plate-holder. — The  plate-holder,  Fig.  40,  is  a  rectangular  frame,  /,  of  some  solid, 
flexible  material,  backed  with  rubber  cloth,  bt  stretched  upon  and  glued  to  the  frame, — the 
sensitized  paper,  which  is  used  instead  of  glass  plates,  when  in  position,  resting  against  it. 


PHOTOGRAPHY  APPLIED    TO   SURVEYING. 


The  shield  s,  of  flexible  card-board  blackened,  fits  into  grooves  in  the  edges  of  the 
holder,  and  is  manipulated  in  the  usual  manner.  The  milled  head,  shown  at  the  left  in 
Fig.  35,  serves  to  release  a  hinged  brass  clamp,  or  cleat,  when  the  holder  is  to  be  inserted, 
and  to  secure  the  holder  after  insertion,  so  as  to  entirely  exclude  the  light.  Several 
plate-holders  are  used,  and  are  easily  packed  for  transportation. 

39.  Scale  of  Azimuths  and  Slopes. — Fig.  41    represents  the  means  by  which  a  scale  of 
azimuths  and  of   slopes  is  impressed  or  marked  upon  the  negative.     Two  notched,  curved 

brass  strips,  h,  having  their  centres  in 
the  axis  of  rotation,  are  attached  verti- 
cally to  the  lid  and  base  respectively,  and 
immediately  in  front  of  the  plate-holder ; 
they  project  slightly  beyond  the  frame 
of  the  latter,  so  that  the  notches  may 
be  represented  on  the  negative  by  the  ex- 
posure. The  strips  v  are  similarly  at- 
tached to  the  edges  of  the  vertical  frame. 
The  notches  in  h  correspond  to  divisions 
of  i  "grade,"  or  of  i°,  as  the  case  may 

be,  to  a  radius  =  the  distance  from  the  plate-holder  to  the  axis  of  rotation  of  the  ob- 
jective; and  the  notches  in  v,  to  T^-g  part  of  this  distance.  Projecting  points  at  / 
mark  the  horizon,  or  HH'.  (Note. — On  account  of  the  blurred  images  thus  produced,  it 
would  seem  better  to  attach  these  strips  made  of  very  thin  metal  to  the  inner  edges  of 
the  holder,  so  that  during  exposure  they  would  rest  in  contact  with  the  paper.) 

40.  Adjustment  of  the  Optic  Centre. — In  order  that  the  image  of  every  point  should  be 
clearly  defined   and   immovable  during  rotation   of   the  objective,   it    is   necessary  that    the 
optic  centre  should    be    in   the   axis  of   rotation,   in  or  near  which  line  it   is  supposed  to 
have  been  originally  fixed  by  the  instrument-maker.     To  test  this  condition :  a  narrow  plate 
of  ground-glass,  ruled  with  fine  vertical  lines,  is  placed  in  the  usual  manner  to  receive  the 
image  of  a  distant    point ;    if  the  image  is  displaced    in  the  direction  of   the  rotation,  the 
rear  plate  of  the  objective-carrier  must  be  advanced  by  means  of  the  adjusting  screws;    if 
in  a  contrary  direction,  the  plate  is  moved    to    the    rear.     In    this  adjustment,   to  preserve 
the   parallelism   of   the    plates,   it    is   necessary  that   the   screws  should    be   turned    equally, 
which  condition  is  assured  by  means  of  indices  on  the  screw-heads. 

41.  The  Manipulation. — To  make  an  exposure:    the  camera,  with  objective  capped,  is 
set  up  and  approximately  levelled ;   the  lid  is  raised ;   the  plate-holder,  containing  the  sen- 
sitized  paper,  is   inserted    and    clamped;    the   screens  are   favorably  disposed,   and    the    lid 
is   closed.     The   camera    is   then    levelled ;    the   alidade   is   adjusted    to  the  85°  division  of 
the  scale;   the  vanes  are   raised,   and,   by  revolving  the  camera,  the  vertical  cross-hairs  are 
aligned  upon  the  object  to  occupy  the  centre  of  the  view.     The  shield  is  then  withdrawn; 
the  alidade  is  adjusted  to  the  o°  of  the  scale;    the  objective  is  uncapped  and  revolved  by 
means  of  the  alidade,  until  the  latter  has  reached  the   180°  division, — the  velocity  of  rota- 
tion being  determined  as  in  ordinary  landscape   photography;    and  the  cap  and  shield  are 
then  replaced.     The  time  of  exposure  is  also  determined    in  some  degree  by  the  positions 
of  the  screens,  being  shorter  when  these  are  placed  in  the  outer  notches  (Fig.  39). 


UN:; 


CYLINDRIC  PERSPECTIVES. 


39 


42.  Device  for  Orienting  the  Views. — A  semicircular  limb,  having  its  centre  in  the  axis 
of  rotation  and  its  radius  less  than  that  of  the  camera,  is  attached  to  the  upper  surface 
of  the  base,  and  is  graduated  into  degrees  or  parts  thereof,  from  90°  to  270°,  correspond- 
ing in  position  to  those  on  half  the  compass-limb.  Two  blocks  are  made  to  slide  along 
this  limb,  each  carrying  a  vertical  spindle  about  i£  inches  in  length;  one  spindle  is  ter- 
minated at  its  upper  extremity  by  an  arrow-head,  and  the  other  by  a  crescent.  Just  be- 
fore closing  the  lid  for  an  exposure,  one  of  these  spindles  is  adjusted  to  the  division  cor- 
responding to  that  marked  by  the  compass-needle;  and  the  other,  to  the  division  differing 
from  it  by  90°.  By  this  obstruction  of  the  deviated  rays,  their  positions  are  marked  upon 
the  negative,  the  arrow-head  indicating  the  N.  or  S.  point,  and  the  crescent  the  E.  or  W. 
point,  thus  furnishing  the  basis  for  the  scale  of  azimuths.  The  division  indicated  by  the 
compass-needle  is  also  recorded  in  the  note-book. 

To  illustrate:    For  the  first  exposure  (see  Fig.  42),  the  compass  indication   is   55°;  the 


First  Exposure. 


Second  Exposure. 


Third  Exposure. 


FIG.  42. 

o°-i8o°  line  being  parallel  to  the  optic  axis,  with  the  o°  to  the  front,  the  direction  of 
the  middle  point  of  the  view  is  N.  55°  W.;  consequently  to  mark  the  N.  point,  the 
arrow-head  is  adjusted  to  the  55°  -J-  180°  =  235°  division,  and  the  crescent  to  the  235°  — 
90°  =  145°  division  of  the  scale  to  mark  the  W.  point.  For  the  second  exposure ;  the 
field  being  170°,  and  the  order  in  which  the  views  are  taken  being  from  left  to 
right:  when  the  camera  is  revolved  170°,  the  compass  indication  is  245°;  the  arrow-head 
is  adjusted  to  this  division  and  will  mark  the  S.  point  on  the  negative,  while  the  cres- 
cent, at  the  245°  —  90°  =  155°  division,  marks  the  E.  point.  The  two  views  cover 
170°  X  2  =  340°;  a  third  view  of  20°  is  therefore  required  to  complete  a  tour  of  the 
horizon.  The  exact  supplement  is  obtained  by  revolving  the  camera  until  the  needle  in- 
dicates 245°  —  20°  =  225°,  when  the  adjusted  arrow-head  will  mark  the  S.  point ;  and  by 
withdrawing  the  shield  a  sufficient  extent,  the  E.  point  may  be  marked  by  the  crescent 
placed  at  225°  —  90°  =  135°.  For  reasons  heretofore  given,  it  is  advisable  to  have  the 
views  overlap,  say.  10°,  in  which  case  the  arcs  of  revolution  of  the  camera  would  be 
correspondingly  diminished.  (Note. — The  additions  and  subtractions  above  required  could 
in  great  measure  be  avoided  by  reversing  the  order  oj  the  numbers  on  the  base  scale.) 
43.  The  Field-work  and  Plotting. — The  field-work  corresponds  in  detail  to  that  de- 


PHOTOGRAPHY  APPLIED    TO   SURVEYING. 


scribed  in  paragraphs  18  to  23  for  plane-perspective  cameras;  any  exceptions  due  to  the 
form  of  the  instrument  readily  suggesting  themselves. 

Fig.  43  represents  a  print  ready  for  plotting.  HH'  is  the  horizon ;  SS  and  WW 
are  verticals  drawn  through  the  S.  and  W.  points  respectively ;  and  z  is  the  azimuth  of 
a  point  o,  65°  from  S.,  or  115°  from  N., — S.  being  usually  taken  as  the  origin.  The  verti- 
cal angle  of  o  is  read  off,  as  a  slope-angle,  on  the  side  scale  ;  and  is  y-^j-,  •  •  yf^, 
y^,  •  •  •,  according  to  the  number  of  divisions  from  H'  to  //. 

In  measuring  the  azimuths,  use  the  mean  of  the  distances  SZ  and  S'Z' ;  and  simi- 
larly for  the  vertical  angles,  use  the  mean  of  Hh  and  H ' h '. 

The  stations  plotted,  a  circle  is  described  with  each  plotted  station  as  a  centre, 
and  with  any  convenient  radius ,  the  azimuths  of  the  different  points,  determined  as  above, 
are  protracted,  and  the  points  are  located  on  the  plot  by  intersections  of  right  lines  drawn 


h 
H 


S' 


Z' 


FIG.  43. 


from  the  plotted  stations  through  the  corresponding  azimuthal  divisions.  It  is  apparent 
that  when  chords,  sines,  tangents,  or  ordinary  protractors  are  used  in  protracting  the 
azimuths,  the  greater  their  radii  the  more  accurate  graphically  will  be  the  results. 

As  to  the  heights ;   the  reference  of  any  point    is    given   by  means  of  the  side  scale, 

H'h'  +  Hh 
e.g.:  Suppose — =  3.5    divisions;     then,    the    reference     required    is   .035  X     the 

OT 

plotted    horizontal  distance  from  the  station  to  the  point. 

44.  The  advantages  claimed  in  the  use  of  this  instrument  are  : 

I.  Since  the  scales  are  already  impressed  in  the  exposure,  measurements  may  be  made 
directly  upon  either  the  negative  or  print ;  the    only  construction    required    being    that    of 
the  right  lines,  as  hh'  and  ZZ'  (Fig.  43),  through  the  images  of  the  required  points ;    but 
on  account  of  distortions,  which  might  take  place  in  printing,  it  is  advisable  to  draw  these 
lines  on  the  negatives. 

II.  No  construction  of    the    focal  distance  is  required  ;  nor  is  it  necessary  to  consider 
it  in  the  field-work,  because  the  apparatus  is  effective  only  when  the  optic  centre  is  in  the 
axis  of    rotation  ;  and,  when  adjusted,  the  focal  distance  is  exactly  equal  to  the  radius  of 
the  cylinder. 

III.  No  distortion  exists  if  the    objective    is    rectilinear,  owing  to  the  screens  limiting 
the  field  to  that  effectively  covered  by  the    lens ;    if    it  does  exist,  the  images  will  not  be 
clearly  defined,  therefore  when  these  are    clearly  defined    there  is    positively  no  distortion. 
In  vertical  planes,  however,  there  may  be  a  slight  distortion  near  the  edges  of   the  field  ; 
but  this  is  of  slight    importance,    since    the    higher    points    are    in    the  sky  portion  of    the 


CYL1NDRJC  PERSPECTIVES.  41 

view,  and  the  lower  points  are  so  near  the  station  that  any  small  error  in  a  correspond- 
ing slope-angle  may  be  neglected. 

IV.  Eccentricity  of  position  in  the  plates  "during  the  exposures  i,  2  and  3  at  any 
station  presents  no  inconvenience,  because  each  view  is  oriented  independently  of  the 
others,  on  the  cardinal  points ;  and,  when  required,  the  compass-readings,  which  are  always 
recorded,  furnish  the  elements  necessary  to  orient  the  three  views  at  the  plotted  station. 
The  three-point  problem  may  also  be  utilized  in  locating  a  station  on  the  plot.  (See  also 
close  of  par.  52.) 

To  give  the  photographic  view  a  natural  appearance,  or  to  represent  the  details  as 
they  would  appear  at  the  station,  the  "  cylindroscope "  may  be  used.  It  is  simply  a 
wooden  semi-cylinder,  of  dimensions  equal  to  those  of  the  cylindrical  part  of  the  camera ; 
the  photographs  are  placed  on  its  interior  surface  and  observed  from  the  middle  point 
of  its  axis. 


SECTION   IV. 


o 


D 


RADIAL   PERSPECTIVES.— INSTRUMENTS,  FIELD-WORK   AND   PLOTTING. 

45.  Photographic  Plane-tables. — If  at  any  station  a  photographic  view  of  the  sur- 
rounding tract  be  represented  upon  a  horizontal  surface,  every  point  of  the  representa- 
tion occupying  its  true  position ;  then  will  the  horizontal  angle  of  any  two  points  be 
measured  by  the  angle  included  by  the  right  lines,  or  radii,  drawn  from  the  plotted 

station  through  the  images  of  these 
points.  This  is  the  principle  of 
construction  of  the  following-de- 
scribed instruments.  As  to  rela- 
tive heights  of  points,  means  for 
their  determination  similar  to  those 
described  for  plane  and  cylindric 
perspectives  are  employed. 

46.  Chevallier's  Camera. — In 
1858,  this  instrument  was  submitted 
by  M.  Auguste  Chevallier  to  the 
consideration  of  the  "  Societe  d'En- 
couragement,"  and  favorably  re- 
ceived ;  and  since  that  date  the  in- 
ventor has  succeeded  in  bringing 
it  to  a  still  higher  state  of  perfec- 
tion. In  its  present  improved  form, 
and  as  manufactured  by  the  optician 
Duboscq  of  Paris,  it  consists  of  a 
circular  dark-chamber,  A,  Fig.  44, 
resting  upon  a  triangular  support,  B, 
which  is  furnished  with  levelling- 
screws  that  bear  upon  the  tripod- 
head,  as  shown.  These  attachments 
are  similar  in  construction  to  those 
of  a  theodolite,  thus  admitting  of 
a  revolution  of  the  chamber  in 
azimuth, — a  tangent-screw,  T,  serv- 
ing for  close  adjustments.  A 

plate-holder,  D,  shown  inserted,  contains  the  sensitive  plate.  The  cover,  or  upper 
plate  of  the  chamber,  which  carries  the  objective  system,  is  movable  about  its  centre,  its 
edge  resting  in  double  grooves  in  the  upper  edge  of  the  cylindrical  rim  ;  its  serrated  edge  is 
geared  to  two  pinions,  which  may  be  engaged  at  pleasure ;  to  one  of  these  pinions  motion 


FIG.  44. 


RADIAL   PERSPECTIVES. 


may  be  communicated  by  means  of  a  simple  clock-work  movement,  C,  thus  ensuring 
uniform  rotation ;  while  the  other,  furnished  with  a  projecting  milled  head,  m,  serves  to 
revolve  the  plate  by  hand  to  any  desired  position.  A  graduated  circle  is  fastened  to 
the  edge  of  the  chamber  immediately  below  the  edge  of  the  plate,  and  diametrically 
opposite  openings  in  the  latter  permit  the  reading  of  arcs  or  angles.  A  small  circular 
level  serves  to  ensure  the  horizontality  of  the  plate ;  at  the  centre  of  rotation  and 
projecting  through  the  milled  head  m',  is  a  sharp  metal  point,  furnished  with  a  spiral 
spring,  which  is  thrust  downward  to  mark  this  centre  on  the  sensitive  plate, — an  oper- 
ation, however,  which  may  be  dispensed  with,  as  will  be  shown ;  m'  also  serves  another 
purpose,  to  be  described. 

The  objective  system  consists  of  the  box  E  and  tube  F,  the  axis  of  the  latter 
and  that  of  the  dark-chamber  being  in  the  same  plane ;  the  tube  contains  the  lens, 
and  the  box  has  at  its  rear  extremity  a  triangular  glass  prism,  with  its  edges  horizon- 
tally disposed.  Rays  from  external  objects  entering  the  tube  at  O  are,  after  devia- 
ation  by  the  lens,  reflected  vertically  downwards  through  a  circular  opening  in  the 
plate,  and  are  focussed  on  the  sensitive  plate ;  this  opening,  as  shown  in  the  figure, 
being  between  the  centre  and  edge  of  the  plate. 

A   circular   disk   with    sectoral   openings    of    different   dimensions  is    suspended    imme- 
diately above   the  sen- 
A     sitive   plate,   its   centre 
being   in    the    axis    of 
B     rotation  of  the   instru- 
ment ;   and,    by   means 
of  indices  on  the  milled 

head  m',  any  desired  sector  may  be  brought 
beneath  the  opening  through  which  the  re- 
flected rays  pass,  thus  limiting  the  field  and 
preventing  it  from  extending  beyond  the  cen- 
tre of  rotation.  Thus,  in  Fig.  45,  X  is  the 
axis  of  rotation  of  the  instrument,  /  and  P 
the  lens  and  prism  respectively,  r  the  sen- 
sitive plate,  and  5  the  screen, — S'  represent- 
ing the  latter  in  horizontal  projection,  and 
with  different  sectoral  openings,  h  and  h'  are 
cross-hairs,  which  by  their  obstruction  of  the 
light  are  represented  upon  the  negative ; 
h  is  coincident  with  a  radius  of  the  screen  ; 
and  h'  is  perpendicular  to  h  at  the  point 
in  which  the  ray,  coincident  with  the  optic 
axis  of  the  lens,  intersects  it  after  reflection 
by  the  prism  :  therefore,  when  operating  by 


FIG.  45. 


"  fixed  sectors "  (par.  47),  all  points,  of  which  the  images  are  located  on  k',  are  on  a 
level  with  the  optic  axis  at  the  station  where  the  exposure  is  made.  Simply  to  mark 
the  trace  of  the  "  principal  plane," — that  containing  the  axes  of  rotation  and  of  the  ob- 
jective,— the  smaller  sectors  are  used;  and  the  smallest,  about  i°,  in  operating  by  "con- 
tinuous movement  "  (par.  48),  in  which  case  h  is  suppressed. 


44  PHOTOGRAPHY  APPLIED    TO   SURVEYING. 

It  is  apparent  that  to  facilitate  plotting,  the  distance  of  the  image  of  h'  from  the 
centre  of  rotation  of  the  sensitive  plate,  should  be  exactly  equal  to  the  focal  distance  of 
the  objective,  and  this  condition  is  assured  by  the  instrument-maker. 

In  addition  to  the  parts  above  enumerated,  a  small  telescope  with  cross-hairs,  and  a 
vertical  limb,  are  attached,  as  shown,  to  the  upper  surface  of  the  box ;  and  are  used  for 
pointing,  and  to  measure  vertical  angles  of  points  that  may  lie  without  the  field  of  the 
objective, — the  latter  case  however  rarely  occurring.  When  the  instrument  is  set  up,  the  optic 
axes  of  the  telescope  and  of  the  camera-objective  are  in  the  same  vertical  plane.  An 
attached  compass  affords  the  means  for  orienting  the  views  in  the  subsequent  plotting, 
and  two  methods  for  this  purpose  are  prescribed ; — one  consists  in  rotating  the  plate, 
after  an  exposure  is  made,  until  a  covered  slit  in  the  base  of  the  compass-box,  and 
which  extends  through  the  plate,  is  in  the  vertical  plane  of  the  needle ;  this  cover  is  then 
removed  for  an  instant,  and  an  image  of  the  needle  is  formed  on  the  sensitive  plate, 
thus  giving  the  required  meridian  line:  as  the  compass  is  located  near  the  edge  of  the 
plate,  the  image  of  the  needle  will  not  interfere  with  other  details.  The  other  method 
consists  in  placing  a  staff  vertical  at  some  distance  in  front,  and  in  the  field  of  view ;  its 
image  in  the  negative,  and  its  bearing  obtained  by  means  of  telescope  and  compass,  afford 
the  necessary  data.  The  instrument  is  permanently  focussed  by  the  maker  for  distant 
views;  and  the  usual  form  of  cap  is  used  in  making  exposures.  In  regard  to  size,  it  may 
be  stated  that  excellent  work  has  been  done  with  a  camera  constructed  for  sensitive 
plates  affording  circles  of  but  4  inches  in  diameter. 

47.  Operating  by  Fixed  Sectors. — This    is   its   least  important  use.      Suppose  the   cam- 
era set  up    at   any  station,  pointed   at  a  distant  signal,  and  an  exposure  made ;   an    image 
of  the  signal   and  its  surrounding  features   is   obtained,  and  the  image  of   h  will  cut  that 
of   the   signal.      If   now   the    plate    be    rotated,  the    chamber   being   clamped,    and    an    ex- 
posure made  toward    a   second    signal,    it    is  evident  that  the   angle   included  by  the  two 
images   of   h   on   the   developed   negative,   is  the  true   horizontal    angle  of  these  signals  as 
measured    at    the    station.      The    image    of   h'   will    in    each   view    intersect    points   of   all 
objects   having   the    same    altitude    as    the    optic    axis.       By    using   a    sectoral   opening   of 
about   10°,  a  view  is  obtained   in  which   the  maximum    error  due  to  distortion   falls  within 
the    limit    permissible    in    the   graphical    construction    of    a    plan.      It    is    unnecessary   to 
point  exactly  at  any  signal,  because  with   rough  pointing  the  image  will  always  be  found 
in  the    vicinity  of  h,  and  all  the  images  of  h  in  the  different  sectors  must  have  their  true 
relative  positions;  and  in  this  property  rests  the  value  of  the  instrument  for  rapid  work. 

48.  Operating  by   Continuous   Movement. — In    this   mode    of  working   lie   the  great    ad- 
vantages  derived    from    its  use,  viz.:    absolute  accuracy  in    the    measurement    of   horizontal 
angles,  and  the  great  rapidity  with  which  the  field-work  may  be  performed. 

In  the  preceding  operation,  a  narrow  sector  had  to  be  used  to  avoid  distortion,  and 
to  complete  a  tour  of  the  horizon  many  views  were  required.  If  with  this  sector  a 
continuous  movement  were  given  to  the  plate,  sector  and  plate  revolving  together,  images 
of  objects  would  be  superposed  and  become  indistinguishable ;  but  by  reducing  it  to  a 
very  narrow  slit,  coincident  in  position  with  k,  the  sensitive  plate  will  receive  at  each 
instant  the  images  of  those  points  only  that  are  situated  in  the  principal  plane  (par.  46) ; 
there  will  be  no  superposition  throughout  a  revolution ;  and,  from  a  view  thus  obtained, 
the  true  horizontal  angle  of  anv  two  points  is  given  by  the  radials  that  intersect 
the  images  of  these  points, 

\ 


RADIAL   PERSPECTIVES. 


45 


If,  in  reference  to  the  required  plan,  certain  sectors  of  the  landscape  are  unimpor 
tant,  it  is  only  necessary  to  cap  the  objective  while  traversing  them  :  but  with  the  rapid 
plates  now  in  use,  no  appreciable  time  is  lost  by  making  the  entire  tour. 

As  already  stated,  the  cross-hair  k  is  suppressed,  while  ti ',  reduced  in  length  practi- 
cally to  a  point,  leaves  its  image  upon  the  sensitive  plate  in  the  form  of  a  circumference 
of  a  circle,  described  with  the  centre  of  rotation  as  a  centre,  thus  indicating  the  true 
horizon  of  the  station. 

It  is  apparent  that  with  an  inverting  objective,  and  the  relative  positions  of  objec- 
tive and  prism  shown  in  Fig.  45,  the  sky  will  occupy  the  outer  portion  of  the  picture. 
Such  a  representation  is  called  "nadiral,"  in  contradistinction  to  one  in  which  the  sky 


FIG.  46. 

occupies  a  central  position,  which  is  termed  "  zenithal :"  the  latter  is  evidently  produced 
by  placing  the  inverting  objective  between  the  prism  and  sensitive  plate,  as  in  the  other 
form  of  this  instrument.  Although  these  representations  possess  the  same  value  for  sur- 
veying purposes,  the  former  presents  the  more  natural  appearance.  In  either  case,  a  radial 
drawn  upon  the  sensitive  plate  represents  the  vertical  of  any  point,  the  image  of  which  it 
intersects. 

49.  The  Field-work  and  Construction  of  the  Plan. — As  in  other  methods  of  surveying, 
a  base,  as  AB,  Fig.  46,  is  measured,  and  from  each  extremity  a  tour  of  the  horizon  is 
made  ;  this  is  repeated  at  a  third  station,  as  C,  which  is  contained  in  views  from  A  and  B ; 
and  so  on  throughout  the  tract.  The  selection  of  stations  is  governed  by  the  conditions 


46  PHOTOGRAPHY  APPLIED    TO   SURVEYING. 

already  stated  in  par.  19.  It  is  apparent,  from  the  usual  multiplication  of  representations 
of  any  point,  that  numerous  opportunities  for  verification  are  afforded  for  the  plotting. 
It  is  hardly  necessary  to  state  that,  in  setting  up  the  instrument,  the  centre  of  rotation 
should  be  plumbed  over  the  station-point,  and  the  axis  of  rotation  made  truly  vertical. 

In  constructing  the  plan,  either  negatives  or  prints  may  be  used  directly  as  pro- 
tractors. On  the  negatives,  the  landscape  is  represented  as  if  rotated  horizontally  180°  ; 
due  allowance  should  therefore  be  made  for  the  position  of  the  N.  point.  On  the 
prints,  in  addition  to  this,  when  the  sensitive  plate  is  exposed  film  up,  the  positions  of  ob- 
jects are  reversed  ;  and  the  remedy  lies  in  either  placing  the  film  down,  or  in  making  the 
prints  transparent  with  oil  or  other  material. 

Fig.  46  fully  illustrates  the  construction  of  the  plan;  the  base  is  plotted  to  the 
required  scale;  the  views  from  the  extremities  are  placed  with  their  centres  at  these 
plotted  points,  and  are  rotated  until  their  meridian  lines  are  parallel  to  the  direction 
assumed  for  the  meridian  upon  the  sheet;  right  lines  are  then  drawn  from  the  centres 
to  corresponding  points  and  produced  to  intersection,  thus  locating  required  points  as  in 
plane-table  surveying.  A  third  view  is  then  similarly  oriented  and  used,  and  so  on 
throughout  the  work. 

50.  Determination  of  the  Heights. — Since   the   image   of   the   circumference   described 
by  the   cross-hair  h'  is   the  horizon  of  the  picture,  it  serves  as  a  datum  line  to  which  all 
points  are  referred:   the  reference  of  any  point  is  therefore  given  by  the  formula 

h 

x  =  md  -f.  ; 

in   which  /  is  the  focal  distance,   -  -  the   scale   of  the  map,  d  the  distance  from  the  sta- 

m 

tion  to  the  point,  measured  on  the  plan,  and  h  the  vertical  distance  of  the  point  from 
the  horizon,  measured  on  the  view.  Since  the  maps,  or  at  least  the  first  sheets  of  the 

m 

respective  maps,  may  all  be  plotted  to  the  same  scale,  -p  may  be  considered  as  a  con- 
stant quantity;  a  convenient  coordinate  table  may  therefore  be  prepared,  giving  the  values 
of  x  for  values  of  d  and  /z,  differing  by,  say,  100  feet  and  10  feet  respectively,  thus 
facilitating  the  plotting.  The  references  to  a  common  datum  plane  are  obtained  in  the 
usual  manner. 

51.  Instrumental  Error,  Advantages    Claimed. — The    errors  are    those    observed    in    all 
optical  instruments  employing  lenses.     Distortions  in  a  horizontal  direction  are  destroyed ; 
while  those  in  a  vertical  direction,  if  existing,  may  be  detected  by  comparison  of  "fixed 
sector"  views  with  those   obtained  by  "continuous  movement,"  and  then  corrected.     The 
only   cause   of   confusion    is   the   slight   angular    deviation   of  a   vertical   ray   after   passing 
through  a  small  sector,    thus   causing    it    to   spread   to   the    right   and  left  of  its  true  posi- 
tion   a   distance    equal    to   half   the   breadth    of   the   sector;    but    this   lack  of    definition  is 
obviated  by  using  a  sectoral  opening  not  greater  than  i°. 

As  illustrating  its  accuracy,  M.  Jouart  states  that,  in  his  own  practice  with  an  instru- 
ment of  this  kind  having  a  focal  distance  of  7.5  inches,  and  in  plotting  to  a  scale  of 
yaViT'  satisfactory  results  were  obtained  up  to  distances  exceeding  I  mile.  In  a  recent 
Parisian  publication,  M.  Gossin  also  states  that  its  use  is  attended  with  excellent  results. 


RADIAL    PERSPECTIVES. 


47 


The  range  of  the  instrument  evidently  depends  entirely  upon  the  power  of  the  objec- 
tive used. 

The  main  advantages  claimed  for  it,  as  compared  with  ordinary  surveying  instruments, 
are  the  certainty  with  which  a  tour  may  be  closed,  and  errors  of  arc-reading  thus  ob- 
viated ;  the  great  rapidity  of  execution ;  its  simplicity  of  manipulation,  which  enables  a 
novice  to  produce  practically  perfect  views;  and,  as  with  all  photographic  surveying  in- 
struments, the  production  of  complete  views  of  the  topographical  features.  Captain  Han- 
not,  chief  of  the  photographic  service  of  the  Belgian  war  department,  states,  in  regard 
to  this  instrument,  that  in  its 
construction  it  embodies  in  a 
most  happy  manner  all  the  im- 
provements of  which  a  camera 
is  capable;'  and  that,  as  com- 
pared with  the  ordinary  plane- 
table,  it  possesses  a  great  num- 
ber of  advantages  in  its  favor. 
With  the  very  sensitive  dry 
plates  now  generally  available, 
and  the  very  narrow  sectoral 
opening  made  permissible  by 
their  use,  joined  to  careful  con- 
struction and  the  employment 
of  a  good  objective,  this  in- 
strument must  prove  of  valu- 
able assistance  in  surveying, 
for  both  civil  and  military  pur- 
poses. 

52.  Mangins  Camera. — This 
instrument,  like  that  just  de- 
scribed, is  of  French  invention. 
It  produces,  without  rotation 
of  any  part,  an  entire  tour  of 
the  horizon  upon  a  single  plate. 
The  theory  of  its  construction 
is  based  upon  the  idea — due 
to  Capt.  Prudent — that  virtual 
images  produced  by  a  spherical  convex  reflector,  might  be  photographed  so  as  to  represent 
the  original  objects  in  their  true  proportions. 

I.  The  Metallic  Reflector. — Col.  Mangin  solved  the  problem,  by  first  constructing  a 
reflector  of  which  the  form  of  the  generating  curve  was  dependent  upon  the  condition 
that  the  virtual  images  should  be  rigorously  exact  for  its  median  zone.  This  reflector.. 
R,  Fig.  47,  is  a  silver-coated  surface  of  revolution,  having  its  axis,  X,  vertical,  in  order  that 
it  may  receive  the  incident  rays  from  all  points  of  the  horizon.  By  total  reflection,  the 
pencils  are  made  to  converge  to  a  point,  O,  of  this  axis;  where  an  objective,  having  its 


FIG.  47. 


48  PHOTOGRAPHY  APPLIED    TO   SURVEYING. 

optic  axis  coincident  with  that  of  the  reflector,  serves  to  produce  upon  the  sensitive 
plate  placed  on  the  base,  B,  of  the  chamber,  the  conjugate  of  an  image  formed  by  the 
reflector. 

For  photographic  purposes,  each  point  of  the  virtual  image  should  be  true  and  clearly 
defined,  and  the  pencil  of  rays  emanating  from  it  rigorously  conical,  in  order  to  produce 
on  B  a  true  image  of  the  original  point.  Neither  a  spherical  nor  a  conical  reflector  would 
serve  this  purpose,  when  the  incident  and  reflected  rays  make  considerable  angles  with 
each  other;  but,  by  making  the  generatrix  an  arc  of  a  parabola,  with  its  axis,  a,  horizon- 
tal, the  incident  rays  in  the  horizon  of  the  station  are  reflected  to  O  as  desired.  In  the 
manufacture  of  the  reflector,  the  arc,  c,  of  an  osculatory  circle,  tangent  at  the  middle  point 


FIG.  48. 

of  the  parabolic  arc,  was  substituted  for  the  latter,  and  the  construction  is  allied  to  that 
of  the  annular  light-house  lens. 

As  the  distance,  measured  on  the  reflector,  from  the  median  zone,  or  middle  horizon- 
tal circle,  increases,  the  definition  decreases ;  but  it  is  found  by  experiment  that  rays,  inci- 
dent upon  any  part,  which  make  angles  of  15°  with  the  horizon,  are  sufficiently  well  de- 
fined for  practical  purposes;  the  vertical  field  of  the  instrument  is  therefore  30°. 

The  distance  from  O  to  B,  and  the  selection  of  an  objective  to  place  at  O,  are  gov- 
erned by  the  rules  observed  in  making  enlargements  by  photography.  For  example :  Let 
d  denote  the  distance  from  O  to  the  median  plane  of  the  reflector;  then,  to  produce  on 


RADIAL   PERSPECTIVES. 
B  an   image  twice   the  diameter  of   the   virtual   image ;    OB  must   be   equal   to   2d,   and   an 
objective  is  used  of  which  the  focal  distance  f  is  derived  from  the  expression  -7.  =  ~j -\-~~j  • 

II.  The  Glass  Reflector. — The  metallic  reflector  is  easily  tarnished  by  chemical  reaction, 
and  is  therefore  hardly  suited  to  photographic  work;  to  remedy  this,  Col.  Mangin  con- 
structed one  of  glass,  of  which  the  form  is  such  as  to  correct  the  effect  of  refraction. 
The  outer  surface,  S,  Fig.  48,  which  receives  the  incident  rays,  is  torus-shaped ;  the  upper 
or  reflecting  surface,  S',  is  of  the  same  form  as  the  metallic  reflector;  and  the  lower  sur- 
face, S",  is  spherical,  its  centre,  Oy  being  the  focus  of  the  reflected  rays:  the  objective  at 
O  is  for  the  purpose  described  in  (I).  This  reflector  has  another  advantage,  since  reflected 
rays  are  not  obstructed  by  the  mountings  which  are  required  for  the  metallic  reflector. 

With  this  camera,  the  diedral  angles  formed  by  vertical  planes  intersecting  the  station 
and  exterior  points,  are  truly  represented  on  the  picture  by  the  sectors  included  by  ra- 
dials  drawn  to  the  images  of  the  points, — the  centre  being  the  image  of  a  minute  open- 
ing, which  admits  the  vertical  ray  R  in  the  axis  of  the  reflector  ;  and  all  points,  in  any 
vertical  plane  of  the  station,  have  their  images  in  the  same  radius.  The  horizon  of  the 
picture,  ////',  may  be  defined,  either  by  the  image  of  a  circumference  engraved  on  the 
reflector,  and  which  is  concident  with  the  line  of  con- 
tact of  a  vertical  cylinder ;  or  independently,  in  which 
case  any  distant  point  in  the  horizon  of  the  station  is 
noted,  and,  with  the  axial  point  as  a  centre,  a  circum- 
ference passing  through  the  image  of  this  point  is  then 
described  on  the  negative  or  print, — checks  are  afforded  - 
by  observing  two  or  more  points. 

The  operation  of  plotting,  from  the  negatives  or 
prints,  is  evidently  similar  to  that  described  for  Che- 
vallier's  camera. 

Fig.  49  represents  the  camera  complete.  In  another  form  of  this  instrument,  a  tri- 
angular prism  is  employed  to  reflect  the  rays,  after  deviation  by  the  objective,  to  the 
sensitive  plate  vertically  disposed, — this  arrangement  making  the  camera  more  compact. 


FIG.  49. 


SECTION  V. 

THE   CAMERA   WITHOUT  A   LENS. 

53.  It  is  well  known  that,  if  rays  of  light  from  exterior  objects  enter  a  dark  chamber 
through  a  very  small  aperture  in  one  of  its  sides,  images  of  these  objects  are  formed  on 
the  side  opposite  the  aperture,  and  that  the  images  will  be  sharply  defined  if  the  aper- 
ture is  circular  and  its  edge  is  free  from  flaws. 

The  apparatus  required  is  either  an  ordinary  camera,  or  a  box  especially  constructed 
for  the  purpose.  For  general  purposes  its  length  when  extended  should  be  at  least  12  inches, 
and  its  lateral  dimensions  such  as  to  admit  an  ordinary  plate-holder  containing  plates  of 
the  required  size.  Means  already  described  are  resorted  to  for  the  setting  up  and  levelling. 
The  position  of  the  aperture  should  correspond  in  every  respect  to  that  of  the  intersection 
of  the  optic  axis  of  an  objective  with  the  front  face  of  a  camera ;  therefore,  to  make 
the  arrangement  complete,  the  aperture  should  be  constructed  in  an  ordinary  objective- 
slide,  of  which  the  centre,  or  flange  for  a  lens-tube,  is  filled  with  some  opaque  material. 
Apertures  of  different  sizes,  corresponding  to  different  fields  of  view  and  sizes  of  images, 
are  required ;  it  is  therefore  more  convenient  for  manipulation  to  construct  them  as 
hereinafter  described,  and  attach  the  plates  to  a  single  metal  strip  which  may  be  held 
and  moved  in  grooves  against  or  across  a  central  opening  in  the  slide,  so  that  any  desired 
aperture  may  be  readily  made  to  face  the  opening;  or  a  revolving  disk  containing  the 
apertures  may  replace  the  strip.  A  superposed  sliding  strip,  or  another  revolving  disk,  will 
serve  as  a  cap  in  making  exposures. 

To  admit  oblique  rays,  either  the  aperture  must  be  conical,  or  else  constructed  in 
very  thin  metal.  In  the  former  case,  a  conical  cavity,  of  which  the  angle  at  the  vertex 
should  be  at  least  90°,  is  drilled  nearly  through  the  metal,  and  the  perforation  is  com- 
pleted with  a  needle-point;  in  the  latter,  the  thin  metal,  platinum,  tin-foil,  or  gold  leaf,  is 
placed  on  a  flat,  smooth,  hard-wood  surface  and  pierced  with  a  needle,  care  being  taken 
in  this  as  in  the  former  case  to  make  the  aperture  circular  and  of  the  required  size  ;  the 
latter  is  easily  determined  with  the  aid  of  a  magnifying-glass  and  a  finely  divided  scale 
of  equal  parts, — a  needle  gauged  and  suitably  marked  will  prove  useful  in  this  operation. 
When  thin  or  leaf  metal  is  used,  it  is  of  course  attached  to  annular  pieces  of  thicker,  or 
less  pliable  material,  before  being  fastened  to  the  strip  or  disk;  and  the  latter  is  suitably 
pierced  so  as  not  to  obstruct  the  rays.  As  a  precaution  against  reflections  in  the  camera, 
metal  parts  facing  the  interior  should  be  blackened. 

To  obtain  well-defined  images  the  sizes  of  apertures,  and  the  distances  from  apertures 
to  the  sensitive  plate,  which  may  be  termed  aperture  distances,  should  vary  according  to  the 
following  rules  derived  from  experiment : 

For  an  aperture  distance  of  3.14  inches,  the  size  of  aperture  is  0.012  of  an  inch  ; 
"  "  "  11.80       "     ,        "  "         "  0.02       "         "     ; 

and  between  these  distances,  the  sizes  vary  proportionally  from  those  given. 

It  is  apparent  from  a  consideration  of  the  limiting  rays  of  any  field  of  view,  or  of 
any  object,  that  the  shorter  the  aperture  distance,  the  wider  will  be  the  field  embraced  in 
a  photograph,  and  the  smaller  will  be  the  images.  A  short  aperture  distance  entails  less 


THE   CAMERA    WITHOUT  A   LENS. 


exposure  than  a  long  one,  because  of  the  greater  illumination  ;  and  a  90°  field  is  found  to 
be  about  the  limit  of  equal  illumination. 

Assuming,  for  surveying  purposes,  90°  as  the  desired  horizontal  field  or  angle  of  view, 
then  in  order  that  any  plate  shall  exactly  contain  the  view,  the  aperture  distance  should 
be  one  half  the  horizontal  dimensions  of  the  plate ;  but  for  reasons  heretofore  stated  a 
marginal  allowance  is  advisable,  therefore  it  is  best  to  make  the  distance  a  trifle  less — 
say  4.75  inches  for  an  8xio,  and  3.75  inches  for  a  5x8. 

In  the  use  of  this  camera,  images  on  a  ground-glass  are  so  faint  as  to  be  dis- 
tinguished with  difficulty,  except  in  the  brightest  light ;  therefore,  to  dispose  the  camera 
for  any  view,  the  following  simple  device  is  employed :  For  the  horizontal  field  ;  mark,  by 
a  projecting  pin  or  otherwise,  a  point  on  the  front  upper  edge  of  the  camera  and  exactly 
in  the  vertical  of  the  aperture  ;  and  mark  two  other  points  on  the  rear  upper  edge,  one 
near  and  opposite  to  each  extremity  of  the  upper  edge  of  the  sensitive  plate  ;  then  the 
prolongations  of  the  sides  of  the  angle,  formed  by  joining  the  front  point  with  the  other 
two,  will  evidently  serve,  by  sighting  along  them,  to  adjust  the  aperture  distance  so 
that  any  desired  field  will  be  contained  by  the  plate.  A  similar  device  on  a  side  of  the 
camera  serves  to  determine  the  vertical  field. 

A  tour  of  the  horizon  requires  but  four  separate  views  of  90°  each ;  and  each  view 
may  be  limited  to  its  proper  quadrant  as  follows :  At  the  first  exposure,  sight  a  distant 
point  in  the  prolongation  of  one  of  the  lateral  faces  of  the  camera ;  then,  assuming  the 
camera  to  be  rectangular  in  plan,  for  the  second  disposition,  revolve  the  camera  until  this 
point  is  intersected  by  the  plane  of  a  longitudinal  face  ;  and  similarly  for  the  two  remaining 
views. 

With  sensitive  dry  plates,  the  period  of  exposure  for  landscapes  is  about  10  seconds  in 
sunlight,  and  from  25  to  35  seconds  on  a  cloudy  day.  With  a  Stanley  dry  plate,  sensitive- 
ness 25,  aperture  0.013  of  an  inch  in  platinum  leaf,  aperture  distance  4.5  inches,  and 
exposure  10  seconds,  the  author  obtained  an  excellent  negative,  which  at  ordinary  visual 
range  could  hardly  be  distinguished  from  one  sharply  focussed  with  a  lens.  The  following 
table  from  successful  experiments  will  prove  useful: 


SUBJECT. 

PLATE. 

APERTURE. 

Ap.  DISTANCE. 

EXPOSURE. 

REMARKS. 

Inches. 

Inches. 

Landscape  

Wet  plate  

O.O2 

II.  81 

10'  to  15' 

Cloudy  weather. 

« 

Dry  plates,  sen- 
sitiveness  25.. 

0.12 

3-M 

10" 

In  sunlight. 

« 

«             « 

0.  12 

3-T4 

25"  to  35" 

Cloudy  weather. 

In  studio  

«             « 

0.02 

12.0 

i' 

Subject   10  ft.  dis- 

tant, well  lighted. 

There  are  certain  merits  due  to  this  method,  viz. :  that,  when  the  camera  is  properly 
set  up  and  levelled,  the  images  formed  are  free  from  distortion,  and  it  may  be  recom- 
mended for  accuracy  and  simplicity.  Its  usefulness  must  of  course  be  determined  by  local 
circumstances  and  conditions,  such  as  the  degree  of  definition  required,  sensitiveness  of 
the  plates,  and  the  kind  of  lens  available  for  other  methods  of  working.  The  principle 
may  be  readily  applied  to  the  production  of  plane,  cylindric,  or  radial  perspectives. 


SECTION   VI. 


TELESCOPIC   AND   BALLOON   PHOTOGRAPHY. 

54.  Telescopic  or  Long-range  Photography.— -This  has  for  an  object  the  photographic 
representation  of  features  beyond  the  range  of  the  ordinary  camera-objective.  The 
means  required  consist  of  a  telescope  attached  to  the  front  of  the  camera,  and  which 
has  its  optic  axis  in  the  prolongation  of  that  of  the  camera-objective  ; — the  latter  ob- 
jective may  be  dispensed  with.  Fig.  50  shows  an  arrangement  of  camera  and  telescope 
for  the  purpose.  Fig.  51  represents  M.  Lacombe's  device  for  attaching  the  telescope; 
after  inserting  the  eye-piece  a,  the  tube-ring  b,  which  has  a  thread  cut  on  its  projecting 
cylinder,  is  screwed  into  the  outer  extremity  c  of  the  objective-tube;  a  diaphragm,  d, 


0 


FIG.  50. 


FIG.  51. 


with  large  opening  preventing  the  eye-piece  from  injuring  the  objective.  Fig.  50  repre- 
sents the  other  arrangements  and  the  manner  of  focussing. 

Another  French  operator,  M.  Mathieu,  dispenses  with  the  metal  ring  b,  using  instead 
a  cylinder  of  stiff  red  cloth,  which  excludes  actinic  rays.  While  making  the  exposure,  he 
envelopes  the  apparatus  with  a  thick  black  cloth,  in  order  to  thoroughly  exclude  all  light 
except  that  which  enters  through  the  telescope.  There  are  two  conditions  indispensable 
to  success :  one,  that  all  light,  except  that  proceeding  from  the  subject  to  be  photo- 
graphed, must  be  excluded  from  the  apparatus ;  and  the  other,  that  the  exposures  must 
be  timed  with  great  care. 

Figs.  52  and  53  are  sketches  from  "  La  Nature "  of  M.  Mathieu's  work,  the  feature 
C  in  the  former  being  magnified  as  shown  in  the  latter,  by  the  means  just  described.  As 
to  the  details  in  this  case:  The  camera  used  was  a  5x7,  and  the  lens  a  No.  2  Darlot. 
The  time  of  exposure  with  an  o.o8-inch  stop,  an  ordinary  dry  plate,  and  a  range  of  0.75 
mile,  was  90  seconds.  Good  negatives  were  obtained  at  a  distance  of  3.75  miles. 

This  method  is  much  like  that  practised  in  taking  lunar  photographs,  and  which  is 
described  in  some  astronomical  treatises.  Its  application  for  surveying  purposes  would 
prove  of  value  for  representing  inaccessible  objects,  such  as  the  peaks  and  precipitous 


TELESCOPIC  AND    BALLOON  PHOTOGRAPHY.  53 

slopes  of  mountains ;  and,  in  military  operations,  when  the    detailed    description    of   a  dis- 


/ 


•^T-jf-y  f— >- 
V       >.%.*•• 


r-'""-"" ."'"'     ^yp^^'^'^  ''"""'' 

"^M::^^^^j^>t^^^^ 

^  ?SP 


mtfe 


•  r4^n~\-rr  "»"t± — ^IQ* 

tn3QXi;,ii3u^— r " 


FIG.   52. 

tant  redoubt,  or  other  fortification  or  position  is  needed. 

55.  Balloon  Photography — Sketch  of  Progress  to  1880. — Visual  command  afforded 
from  great  heights  on  a  clear  day  is  nowhere  so  perfectly  attained,  as  from  the  cage  or 
car  of  a  balloon  a  few  thousand  feet  above  the  earth's  surface;  there  is  no  substructure 
to  interfere,  and  the  country  is  spread  out 
like  a  map  beneath.  The  idea  of  taking 
advantage  of  such  favorable  conditions  for 
photographic  surveying  seems  to  have 
originated,  in  1855,  with  M.  Andraud ;  but 
it  was  not  put  into  practical  form  until 
1858,  when  M.  Nadar,  who,  having  no 
knowledge  of  a  previous  conception  of  the 
idea,  resolved  to  do  no  less  than  make  a 
survey  of  entire  France,  by  means  of  pho- 
tography and  a  captive  balloon.  His  plan 
was  to  ascend  to  a  height  of  1000  metres 
above  points  previously  determined ;  this 
height  would  command  an  area  of 
1,000,000  square  metres,  or  100  hectares 

(about    250   acres) ;   and,  since  at  least    ten  FIG>  ^ 

stations     could    be    occupied    daily,     wea- 
ther  permitting,    1000   hectares  would  be  the  day's  work.       For   military  purposes,  during 
a   campaign,  communication  with  headquarters  was  to  be  maintained  by  means  of   a  box, 


54  PHOTOGRAPHY  APPLIED    TO   SURVEYING. 

sliding  along  one  of  the  cords  which  held  the  balloon,  and  by  which  the  prints  contain- 
ing views  of  the  enemy's  position,  etc.,  were  to  be  dispatched  every  fifteen  minutes. 
Patents  were  secured  both  in  France  and  foreign  countries,  and  he  set  to  work  developing 
his  plan.  The  swaying  of  the  balloon,  lack  of  sufficiently  rapid  plates,  and  the  escape  of 
gas  from  the  envelope,  made  the  manipulation  so  difficult  that  the  photographs  obtained 
were  of  little  value.  Subsequently,  however,  in  1868,  he  succeeded  in  taking  many  useful 
balloon  photographs  which,  considering  the  means  then  available,  were  as  perfect  as 
could  be  expected. 

In  the  War  of  the  Rebellion,  captive  balloons  were  frequently  used  to  obtain  in- 
formation as  to  the  surrounding  country,  the  enemy's  position,  etc.  In  May,  1862,  the 
Union  army  before  Richmond  employed  one,  and  succeeded  in  photographing  on  a 
single  plate  all  the  country  between  Richmond  and  Manchester  on  the  west,  and  the 
Chickahominy  on  the  east  ;  the  rivers,  smaller  streams,  railroads,  marshes,  pine-woods,  etc., 
were  all  represented,  as  well  as  the  dispositions  of  troops.  Two  prints  were  made  from 
the  negatives  thus  obtained  ;  one  of  which  was  retained  by  the  army  commander,  and  the 
other  by  the  aeronaut.  Rectangles  were  ruled  on  them,  the  same  number  and  disposition 
on  each  ;  reference-letters  were  attached  ;  and,  since  in  the  subsequent  ascents  telegraphic 
communication  was  maintained  between  headquarters  and  the  balloon,  the  prints  enabled 
the  aeronaut  to  give  information  of  all  important  events  that  transpired  in  any  rectangle ; 
which  information  in  several  instances  proved  very  valuable. 

At  this  time,  similar  and  successful  use  of  balloons  was  made  on  the  Mississippi  near 
Cairo.  These,  results,  added  to  those  attained  during  the  war  of  1866  in  Bohemia,  led 
to  the  formation  in  France  of  the  French  Society  of  Aerial  Navigation,  in  existence  at 
the  present  date,  and  including  men  of  wealth  and  scientific  attainment :  the  participa- 
tion of  its  member  in  the  Franco-Prussian  War,  particularly  during  the  siege  of  Paris, 
when  both  free  and  captive  balloons  were  employed,  is  well  known.  The  faults  of 
management  and  the  failures  that  occasionally  took  place,  and  these  were  more  especially 
noticeable  on  the  German  side,  were  mainly  due  to  inexperience,  imperfect  apparatus 
and  lack  of  instruction; — in  this  connection,  it  may  be  stated  that  the  Germans,  in  1887, 
added  to  their  army  an  aeronautic  section  of  fifty  men. 

The  Meudon  School  of  Aerostation  was  formed  in  1871  for  the  purpose  of  instructing 
soldiers  in  the  duties  of  aeronauts.  A  commission,  of-  which  Col.  Laussedat  was  the  most 
active  member,  was  appointed  the  same  date  to  study  this  subject,  and  it  devoted  es- 
pecial attention  to  balloon  photographic  surveying. 

In  1878,  M.  Dagron  undertook  to  carry  out  the  scheme  proposed  by  M.  Nadar  ;  but 
the  slow  action  of  wet  plates,  joined  to  oscillations  of  the  balloon,  resulted  in  con- 
fused negatives ;  and  it  was  only  on  a  very  calm  day  that  he  finally  succeeded  in  ob- 
taining a  fair  negative  of  part  of  the  city  of  Paris,  in  which  most  of  the  prominent 
features  could  be  recognized. 

In  the  following  year,  dry  plates  and  the  instantaneous  shutter  were  used  by  M. 
Triboulet ;  but  after  making  an  exposure  in  a  free  balloon  at  a  favorable  altitude  of  500 
metres,  a  storm  caused  a  descent  into  the  Seine,  and  custom-house  officers  opened  his 
plate-holders  to  examine  for  contraband  articles. 

In    1879,  on  account  of    services    rendered  in  Afghanistan    and  Zululand,  the  English 


V 


TELESCOPIC  AND  BALLOON  PHOTOGRAPHY. 


55 


war    department    made    ballooning  apparatus    part    of   their  permanent  military  outfit,  and 
provided    for    the    instruction    of    troops    in    its    use. 

56.  Recent  Experiments. — In  June,    1880,   M.   Desmarets,  with  rapid  dry  plates  and  an 
•electric    instantaneous    shutter,    obtained    two    good    negatives.      The    first    represented    a 
portion    of    the    village    of    Mesnil-Esnard    near    Rouen,     including   an   area    900   metres 
square,  to  a  scale  of  about  j-yVo  5  the  altitude  of  the  balloon  was  about   noo  metres,  the 
velocity  of  ascent  6  to  8  metres,  and  the  weather    misty.      Although  lacking  in  clearness, 
buildings,    roads,    trees,  and    piles   of   stones    intended    for   paving   purposes,  were    defined. 
The  camera  was  so  disposed  that  the  objective-tube  projected  through  an  opening  in  the 
base  of   the  car.      The  other   negative  was  made   at    an  altitude  of    1300  metres,  and  em- 
braced   the    country   between    Rouen    and  Quillebceuf ;    unfortunately  clouds   intervened  in 
many  places ;    but,  by  an    enlargement    of   the    original,   M.  Carette    unexpectedly  brought 
out  many  details,  which  before  were  unobservable  with  a  microscope.      In  the  second  op- 
eration, the  camera   was   secured    to   the    side    of   the    car,  altitudes  were  measured  with  a 
very  sensitive  aneroid  barometer,  and  the  objective  used  was  a  Derogy  8X10,  with  a  focal 
distance  of    11.5  inches;    and,  since  the   time  of   exposure  was  about  ^  of   a   second,  and 
the   velocity    of   translation    of    the    balloon    about    6 

to  7  metres  per  second,  the  angle  of  displacement 
— about  8" — was  insensible.  This  experiment  at- 
tracted much  attention  and  therefore  gave  the  sub- 
ject greater  prominence. 

In  1883,  Mr.  Shadbolt  of  England  succeeded  in 
making  with  dry  plates  good  negatives  from  ex- 
posures at  altitudes  varying  from  500  to  1000  metres. 
In  all  the  prints,  the  topographical  features  were  dis- 
tinguishable ;  but  in  one  obtained  at  650  metres 
above  Stamford  Hill,  in  the  northern  part  of  London, 
the  features  were  particularly  well  defined ;  in  fact, 
with  the  aid  of  a  microscope,  the  smallest  objects 
could  be  easily  recognized.  Experiments  of  this 
nature  are  now  being  made  at  the  school-  of  bal 
looning  at  Chatham. 

57.  Special  Apparatus  for   Balloon   PJiotography. — 
In  the  instances  given,  the  aeronaut  occupied  the  car 
and   manipulated    his    camera    in   the   usual  way;    but, 
in   1884,     M.    Triboulet     devised    another    method    of 
working.     He  constructed    a  hexagonal   willow    basket 
to  contain  the  photographic  apparatus,  and  suspended 
it  beneath  the  car  of    a    captive   balloon.     The    latter 
was    unaccompanied    by    an    aeronaut, — electric    com- 
munication with  the  earth,  by  means  of  a  light  cable,  FIG.  54. 
serving  to  make  the  exposures.     As  shown  in  Fig.  54, 

the  interior  of  the  basket'  was  subdivided  into  seven  dark  chambers  ;  six  of  the  objectives 
were  disposed  horizontally  to   obtain    a    tour  of   the   horizon,  while   the  seventh   projected 


PHOTOGRAPHY  APPLIED    TO   SURVEYING. 


through  the  base  of  the  basket  and  gave  the  view  beneath.  Sensitive  dry  plates  about 
8XIO  inches  in  dimension  were  used,  the  plate-holders  occupying  the  positions/;  and  the 
images  on  the  plates  overlapped  0.4  of  an  inch  to  facilitate  joining  the  prints.  The  in- 
stantaneous shutter,  drop-pattern,  was  arranged  as  shown  in  Fig.  55  ;  the  drop  a,  held 
in  the  grooved  strips  b,  was  retained  in  position  by  the  stop  c  attached  to  a  metal  spring 
d\  on  closing  the  circuit  the  electro-magnet  e  withdrew  the  stop,  and  the  drop  was  quickly 
closed  by  means  of  the  rubber  bands  /.  The  photographs  obtained  in  this  instance 
were  fairly  good. 

At  the  close  of  1884,  M.  Casse  made  use  of  a  very  small  captive  balloon;  and, 
to  avoid  the  weight  due  to  the  use  of  an  electric  cable,  substituted  slow-match,  which 
being  of  a  length  corresponding  to  the  period  of  ascent,  released  the  stop  (c,  Fig.  55) 
at  the  proper  elevation.  Clock-work  has  also  been  employed  for  this  purpose.  Experi- 


L 

a             c 

'OH 

o 

- 

XX 

b 

/\ 

b 

f                 \ 

\ 

f/ 

\ 

FIG.   55. 


FIG.  56. 


ments   of   this   nature  and    attended    with    similar   results   were  made   at    Chatham   during 
the  same   year. 

58.  Notably  Successful  Experiments. — A  notable  instance  is  afforded  in  the  experience 
of  MM.  Tissandier  and  Ducom  at  Paris  in  1885.  The  ascent  was  made  at  1.40  P.M.,  in 
a  partially  cloudy  atmosphere,  and  with  a  wind-velocity  of  about  12  miles  per  hour.  The 
photographic  apparatus  consisted  of  a  5  X  8-inch  camera,  Mackenstein  tourist  pattern,  at- 
tached to  the  car  as  shown  in  Fig.  56  ;  and  a  Francois  rapid  rectilinear  objective  No.  4, 
having  a  22-inch  focus  and  an  opening  of  1.4  inches,  stopped  to  I  inch.  The  drop- 
shutter  used  gave  an  exposure  of  -^  of  a  second,  and  the  gelatino-bromide  plates  were 
specially  manufactured  for  the  purpose.  Exposures  were  made  at  altitudes  varying  from 
600  to  iioo  metres;  several  were  very  successful,  the  best  appearing  to  be  that  at 
600  metres  above  the  isle  of  Saint-Louis,  and  in  which,  with  the  aid  of  a  microscope,  the 
coils  of  rope  contained  in  the  passing  boats  are  plainly  visible — a  tracing  from  the  pho- 
tograph, in  the  author's  possession,  is  given  in  Fig.  57.  The  most  sharply-defined  views 
were  obtained  when  the  sun's  rays  were  so  oblique  that  slight  shadows  weie  cast.  The 
motion  of  translation  did  not  affect  the  sharpness,  but  it  was  found  best  to  spring  the 
shutter  near  the  beginning  or  end  of  an  oscillation. 


TELESCOPIC  AND   BALLOON  PHOTOGRAPHY. 


57 


In  July  of  the  same  year,  M.  Pinard,  of  Nantes,  obtained  good  7X9  negatives 
from  exposures  made  at  altitudes  varying  from  400  to  1700  metres.  He  used  a  10- 
inch  focus  Steinheil  antiplanat,  with  a  stop  of  y1^,  and  the  drop-shutter  gave  an  expo- 
sure of  -gJ-Q-  of  a  second. 

The  latest  and  best  results  appear  to  be  from  the  experiments  of  Captains  C.  and 
P.  Renard  and  M.  Georget  in  1885,  and  of  MM.  Tissandier  and  P.  Nadar  in  1886.  Of 
the  many  good  photographs  made  by  the  former  party,  that  one  obtained  at  10.17 
A.M.,  at  an  altitude  of  720  metres,  deserves  especial  mention  for  fineness  of  detail.  The 
latter  party  of  aeronauts  made  an  ascent  lasting  6  hours,  and  obtained  30  photographs, 
including  some  from  an  altitude  of  1200  metres;  various  dispositions  of  the  camera  were 


FIG.  57. 


made    in   order   to    obtain  both  plan    and    oblique   views.      The  time  of  exposure  was 
of  a  second. 

59.  Photographic  Outfit  and  Method  of  Working.  —  The  following  is  a  brief  descrip- 
tion of  Mr.  Shadbolt's  preparations  for  balloon  photography  :  "  My  list  of  requisites  com- 
prises a  5X7i-inch  camera,  a  good  supply  of  double  plate-holders,  lenses,  shutters,  etc.,  an 
aneroid  barometer  with  altitude-scale  registering  up  to  15,000  feet,  a  pocket-compass  and 
maps  of  the  country  over  which  I  am  to  travel,  and  a  copy  of  Murray's  threepenny 
time-tables,  small  edition,  which  comes  in  handily  on  landing.  If  circumstances  permit,  I 
prefer  to  fix  my  apparatus  before  starting  ;  —  the  case  containing  the  slides,  etc.,  being 
attached  to  the  .  side-ropes  of  the  car  by  means  of  a  strap,  so  as  to  be  close  at  hand, 
and  the  camera  fixed  in  its  place  on  the  edge  of  the  basket  by  means  of  any  simple 
device.  .  .  .  The  barometer  I  fasten  to  the  rigging  close  by  the  camera,  so  as  to 
enable  me  to  record  the  altitude  as  each  exposure  is  made." 


5 8  PHOTOGRAPHY  APPLIED    TO   SURVEYING. 

60.  Use  of  Small  Balloons,  Captive  and  Free. — In  1881,  Capt.  Elesdale,  R.E.,  pro- 
posed to  use  a  small  balloon,  either  captive  or  free,  having  sufficient  lifting  power  to  carry 
a  very  light  camera  to  any  desired  height ;  the  exposure  to  be  made  at  the  proper  time 
either  by  electric  apparatus  controlled  from  the  ground,  or,  automatically,  by  clock-work- 
attachment.  In  his  subsequent  experiments,  various  devices  were  employed  to  ensure 
a  proper  direction  of  the  optic  axis ;  for  plan  views,  the  axis  was  kept  vertical  by  at- 
taching the  camera,  pointing  downward,  to  a  rectangular  frame,  suspended  horizontally 
from  a  ring  beneath  the  balloon  by  means  of  four  cords  or  steel  chains  of  equal 
length ;  for  inclined  views,  the  optic  axis  was  to  be  pointed  in  any  desired  direction 
by  attaching  the  camera  to  one  end  of  a  bamboo  rod,  the  retaining  or  ground  cord 
being  secured  to  the  other  end,  while  the  rod  was  suspended  at  the  required  angle  by 
two  cords  of  different  lengths;  a  light  sail,  formed  by  filling  in  the  triangle  of  the  rod 
and  cords  with  silk,  was  suggested  as  a  means  for  keeping  the  camera  steady  in  the 
direction  of  the  wind.  In  case  of  no  wind,  a  multiple  camera  similar  to  that  used 
by  M.  Triboulet  (see  par.  57),  was  to  be  employed. 

For  military  purposes,  as  in  the  case  of  an  investment,  the  besieger  could  resort  to 
either  one  of  the  two  following  processes  for  obtaining  comprehensive  views  of  the 
enemy's  position  : 

a.  A    line   of   connected    balloons  is   to    be   floated    across  the  position,  and  the  views 
are   to    be    taken    in    transit   as    follows :    Having   selected    a   suitable   point   to   windward, 
its   location    being   accurately   ascertained    by   the   preliminary    use    of   one    or   more    pilot- 
balloons,    balloon    No.    I    with   camera   attached    is   sent    up,    its   retaining-cord    of   silk,    or 
best   hemp,  being   paid    out  freely   from    a   windlass   running    on    friction-rollers,    so   as   to 
offer   as   slight    a   resistance   as   possible.       No.  I   having  reached  its   equilibrium   level,  say 
2000   feet,  No.  2    is   instantly   toggled    on    and    sent    up ;    then    No.  3,  and    so   on    through- 
out   the   series   of   five   or   more ;    the    interval,    or   section    of   cord    between    balloons,  evi- 
dently  depending   upon    the   lifting   power.     No    check    of   ascent    is   to    be    permitted,    or 
the   series   will    tend   to   pivot   around    the  starting-point    and  come    to  the    ground.     The 
automatic    apparatus    is   set   at  the    proper   time    for   making   an   exposure,    from    observa- 
tion of   the    rate   of   travel  of   the    pilot-balloons ;  or,  for  the    later   balloons  of   the  series, 
from  information    communicated    by    signals   from     an     assistant,    stationed    in    a    direction 
perpendicular  to    the   path,    and    who   would    observe    the  leading    balloons. 

The  system  is  brought  to  the  ground  beyond  the  enemy's  position  by  attaching 
a  very  light  grapnel,  with  a  suspension  cord  50  to  100  feet  in  length,  to  the  last  bal- 
loon ;  and  by  cutting  a  small  hole  near  the  crown  of  the  latter  to  permit  the  gradual 
escape  of  its  gas,  which  will  begin  to  be  effective  when  the  balloon  has  reached  its 
equilibrium  level. 

b.  In  the  other  process,  each    balloon  is  to   be   sent    up    independently,  the    escape    of 
gas   being   so  provided   for,  as   above,  that  the  balloon  shall  descend  at  a  suitable  distance 
and    be   held    by  its  grapnel. 

In  a  special  case,  as  in  the  close  investment  of  a  fortress  when  it  is  necessary  to 
ascertain  if  an  inner  line  of  defence  is  being  constructed  by  the  besieged,  two  methods 
are  prescribed,  the  choice  depending  upon  the  direction  of  the  wind.  First,  when  the 


TELESCOPIC  AND   BALLOON  PHOTOGRAPHY.  59 

wind  is  favorable  and  blowing  towards  the  enemy:  the  small  balloon  is  inflated  in  the 
advanced  trench,  say  at  200  yards  from  the  work;  it  is  sent  up,  with  camera  attached 
to  a  rod,  as  above  described ;  the  view  is  ta"ken  at,  say,  80  yards  from  the  work,  when, 
by  the  release  of  a  weight  suspended  from  the  balloon  by  a  cord,  which  is  automati- 
cally parted  by  the  burning  of  a  time-fuze,  the  balloon  rapidly  ascends  out  of  reach 
of  the  enemy's  bullets;  the  windlass  is  then  quickly  carried  to  the  rear  trenches,  the 
cord  being  paid  out  freely,  and  the  balloon  is  hauled  down  at  a  safe  distance.  Pro- 
vided the  camera  is  not  hit,  a  few  bullets  through  the  envelope  do  no  harm.  In 
the  second  case,  when  the  wind  is  from  the  opposite  direction:  a  solid  shot,  to  which 
a  short  length  of  chain  connected  with  a  fine  hempen  line  is  attached,  is  fired  with 
the  proper  powder-charge  to  a  point  near  the  crest  of  the  glacis ;  the  balloon,  fastened  to 
the  line  and  also  to  its  retaining  cord,  is  then  sent  up;  the  line  is  burned  through  at  the 
proper  time  by  a  time-fuze,  the  balloon  rises  rapidly  and  is  hauled  down  as  before. 

To  ensure  steadiness  of  ascent  and  to  prevent  oscillation  of  the  captive  balloons, 
Capt.  Elesdale  prescribes  that  "  all  vertical  strain  except  that  due  to  the  dead  weight  of 
the  cord  itself  should  be  removed ;  and  all  horizontal  strain  should  be  avoided  if  possible, 
and  when  this  is  not  possible  it  should  be  changed  into  a  small,  steady  and  uniform 
retarding  force,  instead  of  an  unyielding  pull,  until  the  picture  is  taken."  In  case  of 
the  balloon  rising  to  still  air  above  a  wind-current,  to  keep  it  in  position  "  the  drift- 
ing lower  portion  of  cord  must  be  fed  with  more  cord  from  the  ground  as  fast  as  it 
will  run  out." 

As  a  result  of  his  experiments  up  to  August,  1883,  Capt.  Elesdale  remarks  "that 
with  a  proper  and  thoroughly  worked  out  apparatus,  constructed  by  the  aid  of  opticians 
and  mathematical-instrument  makers,  so  as  to  admit  of  instrumental  accuracy  in  all  the 
processes,  and  for  pictures  taken  on  ordinary  ground  (not  representing  any  violent  slopes 
or  extreme  differences  of  level)  we  may  expect  to  attain  by  balloon  photography  more 
accurate  plans  than  are  usually  obtainable  by  ordinary  surveying." 

61.  Observations  on  Balloon  Photographic  Surveying. — The  facts  given  in  paragraphs  55 
to  58  show  that  good  photographs,  giving  either  vertical,  oblique  or  panoramic  views 
of  a  country,  may  be  obtained  from  balloons ;  and  the  first  question  now  presented  appears 
to  be  whether,  aside  from  their  value  as  pictorial  representations,  they  may  be  used 
as  data  for  the  construction  of  exact  or  true  maps. 

I.  With  Captive  Balloons. — Taking  first  the  case  of  a  single  view:  When  a  balloon 
in  mid-air  is  stationary,  as  in  the  case  of  a  captive  balloon  in  still  weather,  a  panoramic 
view  obtained  corresponds  to  cases  described  in  the  preceding  sections  ;  with  a  difference 
in  its  favor,  however,  that  the  point  of  observation  is  exceptionally  commanding.  The 
vertical  or  plan  view — that  obtained  by  pointing  vertically  through  the  base  of  the  car — 
affords  under  favorable  illumination  a  perfect  means  for  the  measurement  of  horizontal 
angles  ;  the  middle  point  of  the  view  being  the  radial  or  angular  point.  A  vertical  view 
of  a  flat  tract  would  be  a  most  perfect  and  detailed  map  or  plan  of  itself,  and  could  be 
enlarged  or  reduced  as  desired  to  conform  to  any  scale, — the  plans  of  buildings,  which, 
by  their  positions  in  the  outer  part  of  the  view,  would  present  one  or  more  faces,  being 
given  by  the  bases ;  but  in  the  case  of  a  mountainous  or  hilly  tract,  neither  the  eleva- 


6o 


PHOTOGRAPHY  APPLIED    TO   SURVEYING. 


tions  nor  horizontal  distance  would,  as  a  rule,  be  truly  represented.  Thus,  as  shown  in 
Fig.  58 :  at  the  altitude  PB,  the  peak  D  is  projected  by  a  visual  ray  in  D'  instead  of 
D",  representing  an  error  of  D" D'  in  its  horizontal  distance  from  P.  Both  the  distance 
PD"  and  the  elevation  D" D  may  however  be  readily  determined  as  follows :  On  the 

print,  measure  the  angle  @,  which  is  given  by  its 
tangent,  PD1 ', — PB  being  measured  by  the  aneroid  ; 
from  the  station  P  measure  the  angle  of  elevation 
@' ;  plot  the  triangle  PDB,  in  accordance  with  the 
scale  of  the  map;  then  D" D  and  PD" ,  measured 
by  this  scale,  are  respectively  the  elevation  and 
horizontal  distance  required.  For  oblique  views,  the 
plotting  would  be  more  complex — the  measured 
angles  requiring  the  correction  due  to  the  angular  dis- 
placement of  the  optic  axis  from  a  vertical.  It  has 
been  suggested  that  compensation  for  obliquity  of 
the  plane  of  the  picture,  due  to  either  uniformly 
sloping  ground,  the  optic  axis  being  vertical ;  or  to 
an  oblique  position  of  the  axis,  in  the  case  of  flat 
country ;  may  be  effected,  as  in  copying  by  pho- 
tography, by  a  simple  adjustment  of  the  original 
photograph,  or  of  the  sensitive  plate,  in  a  secondary 
photographic  process. 

When  the  captive  balloon  is  not  stationary,  and 
even  when  oscillating  under  a  strong  wind,  it  is  shown,  from  experiments  made  at 
Berlin  in  1885,  that  the  car  itself  can  be  maintained  with  its  base  horizontal.  The  cords 
of  the  net,  instead  of  converging  in  the  usual  manner  to  a  ring  beneath,  were  at- 
tached at  intervals  to  a  horizontal  bar.  For  the  balloon  used, — a  sphere  of  50,000  cu.  ft. 
capacity,  filled  with  hydrogen,  and  carrying  eight  persons, — the  bar  was  2  inches  in  diam- 
eter and  33  feet  in  length,  and  the  car  was  suspended  from  its  middle  point  ;  two 
light  cables  0.4  inch  in  diameter  were  fastened  one  to  each  extremity  of  the  bar,  and 
were  joined  65  feet  beneath  it  to  the  main  cable.  Suppose  the  balloon  to  be  moved  out 
of  the  vertical  of  the  station  ;  then,  if  the  sensitive  plate  is  horizontal,  the  view  itself 
will  show  the  intersection  of  the  optic  axis  with  the  earth ;  this  point  might  also  be 
fixed  by  bearings  of  known  points  taken  from  the  car,  or  by  transit  measurements  from 
the  earth, — the  instant  of  exposure  in  the  latter  case  being  communicated  by  signal. 

If  a  series  of  views  are  thus  taken  above  known  points  of  an  extensive  tract, 
they  may  be  used  in  plotting  as  so  many  protractors,  by  fastening  their  centres  with 
needle-points  to  the  plotted  stations ;  then,  by  orienting  them  and  conforming  to  the 
plane-table  method  of  intersections,  the  required  points  are  readily  located.  A  scale  for 
any  single  view  is  easily  constructed  from  the  ratio  of  any  represented  distance  to  the 
corresponding  actual  distance.  For  the  general  plot,  since  the  method  of  intersections 
is  independent  of  the  diameters  of  the  views,  differences  of  altitude  would  not  affect 
the  construction  of  a  plan,  it  being  only  necessary  to  fix  the  stations  to  scale. 

From  an  altitude    of   2000  feet    excellent   photographs  have  been  obtained ;    and  each 


FIG.  58 


TELESCOPIC  AND   BALLOON  PHOTOGRAPHY.  61 

of  these,  taken  with  a  Hermagis  90°  lens,  would  represent  an  area  about  £  of  a  mile 
square  ;  while  from  a  3250-^.  altitude,  at  which  good  results  are  noted,  the  area  would 
be  about  i^-  miles  square. 

II.  With  Free  Balloons. — But  few  experiments  in  photographic  surveying  with  free  bal- 
loons have  up  to  the  present  date  been  recorded  ;  but  the  proposition  seems  by  no 
means  infeasible,  since  the  dirigibility  of  balloons  has  become  practicable.  In  the  experi- 
ments of  Captains  Templar,  Lee  and  Elesdale,  at  Woolwich  in  1879,  the  differences  in 
direction  of  the  air-currents  at  different  heights  were  utilized  to  travel  in  any  desired 
direction  ;  a  small  pilot-balloon  of  about  200  cu.  ft.  capacity  being  made  to  ascend  or 
descend  a  distance  of  2000  feet  from  the  car,  to  show  the  directions  of  the  air-currents. 
At  Meudon,  in  1884,  Capt.  Renard,  with  a  balloon  propelled  by  means  of  an  electric 
motor,  the  screw  being  at  the  bow,  and  steered  with  a  rudder-sail,  travelled  at  will  in  a 
9-mile  wind,  at  the  rate  of  14  miles  an  hour,  returning  after  a  trip  of  47  minutes  to  the 
point  of  departure.  It  is  evident  that,  in  either  of  these  cases,  vertical  views  taken  at 
proper  intervals  would  have  furnished  a  map  of  the  entire  country  passed  over.  More 
recently,  Captains  Renard  and  Krebs  have  perfected  an  electric  motor  which  ensures  a 
continuous  voyage  of  four  or  five  hours'  duration. 

For  the  purpose  of  measuring  altitudes  with  precision  in  balloon  surveying,  M.  Casella 
has  recently  constructed  a  very  sensitive  aneroid  barometer  which  indicates  differences  in 
height  of  one  foot. 

With  very  sensitive  plates  and  rapid  shutters,  the  photographic  operations  are  of  easy 
performance,  and  photographs  are  obtained  that  lack  nothing  in  clearness  and  accurate 
representation.  Much  assistance  is  afforded  to  military  art  in  the  rapidity  and  faithfulness 
with  which  the  enemy's  defences  and  positions  may  be  depicted  ;  and  to  the  geographer 
the  means  are  supplied  for  obtaining  descriptions  of  isolated  tracts  and  of  those  inac- 
cessible by  ordinary  means.  Panoramic  and  plan  views  well  combined  give  results  of 
great  interest,  and  present  to  the  aeronaut  and  photographer  new  and  important  applica- 
tions of  their  arts. 


SECTION   VII. 

VARIOUS   APPLICATIONS    OF    PHOTOGRAPHIC    SURVEYING,   AND   SOME    OF    ITS 

ADVANTAGES. 

62.  General   Considerations. — It    has   been    shown    in    the     preceding   pages   that    with 
the  camera  as  a  surveying  instrument  the  field-work  may  be  performed  with  great  rapidity, 
and  with  an  economy  of  men  and  material  exceeding  that  due  to  other  means ;  and  that, 
in    the    plotting,  the   plan    results   from,    an    application   of   the   very  simple    method    of  in- 
tersections;  while  the  representation   of    heights    and    surface-forms  is  accomplished  either 
mechanically,   or  by  the  use  of  tabular  values  which    may  be    easily  and    rapidly  applied. 
As  to  the  contouring,  it  is  apparent    that    in  some  cases,  owing  to  the  surface-covering  of 
trees  and  the  interposition  of  projecting   spurs   or  salients,   the  lesser  inequalities  are  not 
given    in    the    prints  ;    but   this    deficiency  can    be  supplied,  as  in  other  methods,  by  field- 
sketching  ;    and,  in  respect  to  this  part  of  the  work,  it  may  be  said  of  this  method,  as  of 
ordinary  topographical  surveying,  that  the    production  of   a  complete   map  requires  a  true 
appreciation  of  forms    of   ground.      With    carefully-constructed    instruments,    and    most    of 
the    standard    cameras    and    rectilinear   lenses  now  made  are  suitable,  the  results  exceed  in 
precision  those  ordinarily  obtained  with  either  compass  or  plane-table;  and  as  to  thorough- 
ness   of   detailed  description,  there"  is  of  course  nothing   to   exceed    a   photographic    repre- 
sentation. 

63.  To   Farm-surveys   and  Engineering  Purposes.— -For    farm-surveys,  fields,  and     small 
tracts    in    general,  a    few    exposures — in    many  cases    three  will    suffice — from    well-selected 
points  furnish  the  data  for  a  complete  map. 

For  engineering  purposes,  aside  from  its  adaptation  to  the  topographical  survey  of 
projected  routes  of  railroads,  canals,  and  other  subjects  which  have  already  been  referred 
to,  this  means  is  especially  useful  for  conveying  information  as  to  the  condition  of  impor- 
tant constructions.  Photographs  obtained  at  different  stages  in  the  progress  of  the  work 
in  hand  afford  descriptions  which,  for  thoroughness,  written  communications  are  inadequate 
to  give  ;  these  faithful  records,  representing  the  site,  foundations,  intermediate  conditions 
of  the  structure ;  with  date,  number  of  men  at  work,  state  of  the  weather,  etc.,  noted  on 
the  back  of  each  print,  are  very  valuable  for  future  reference. 

64.  To  Geographical  and  ArcJiceological  Descriptions. — As  an  aid  to  the  geographer  this 
means  is  of  great  value.      The  difficulties  due  to  optical  illusion  which,  in    ordinary  topo- 
graphical   sketching,  mountainous    or    hilly  tracts    present,  are    obviated  ;    from    the  peaks, 
panoramic  views  are  comprehensive  and  far-reaching ;  and  exposures  from  the  less  elevated 
points  furnish  details  of  the  deeper  valleys,  and  of  those  tracts  which  from  the  peaks  are 
either  shut  out  or  obscure.      The  choice  of  illumination,  properly  exercised,  gives  a  defini- 
tion to  surface-lines    and    points,  that  actually  appears    sharper    in  a  print  than  in  nature ; 


VARIOUS  APPLICATIONS   OF  PHOTOGRAPHIC  SURVEYING.  63 

therefore,  as  compared  with  ordinary  observation,  a  more  accurate  map-delineation  results. 
Both  geological  .and  botanical  information  are  included  in  the  print, — the  rock-structure, 
peculiar  formations,  deposits,  the  forest  and  individual  growths,  and  much  else  that 
assists  to  a  thorough  knowledge  of  the  country. 

Profile  views  of  a  range  of  mountains  or  hills,  taken  either  from  peaks  or  favorable 
points  of  a  valley  route,  are  excellent  data  for  plotting.  The  directions  of  stations  from 
each  other  are  ascertained  by  compass-bearings ;  while  the  aneroid  would  prove  of  valuable 
assistance  in  the  levelling,  and  the  odometer  would  measure  the  distances  between  stations 
of  the  valley  route.  Coast-lines  also  may  be  accurately  described  from  views  taken  off 
shore.  For  the  representation  of  inaccessible  objects,  tracts,  summits,  declivities,  mo- 
rasses, or  craters,  the  method  is  unrivalled. 

To  the  archaeologist  its  use  is  of  inestimable  value ;  intricate  lines  of  figure  of 
buildings,  monuments,  or  earth  constructions,  are  faithfully  produced,  and  their  measure- 
ment is  thus  made  easy  of  accomplishment.  The  photographic  survey  of  Persepolis 
and  of  the  Mosque  of  Mesdjid-a-djunia,  by  Dr.  Stolze,  are  recent  instances  of  its  suc- 
cessful application. 

As  addenda  to  plotted  maps,  the  photographs,  for  minute  information,  will  often 
serve  the  same  purpose  as  actual  inspection. 

65.  To  Military  Purposes, — The  rapidity  with  which  ground  may  be  represented 
offers  special  advantages  to  the  military  service.  As  an  instance  of  rapid  work :  In 
1874,  M.  Javary  made  in  one  day,  in  the  valley  of  1'Arly,  a  photographic  recon- 
naissance extending  14  miles,  and  embracing  the  features  within  a  distance  of  from 
one  to  two  miles  on  either  side  of  the  route  followed.  This  work  was  performed  with 
two  cameras  of  the  ordinary  type,  a  compass  and  a  stadia.  The  two  last  instruments 
were  kept  on  the  route,  while  a  photographer,  with  stadia-rod  and  camera,  marched  on 
each  side.  At  a  given  signal,  the  distance  to  either  photographic  station  was  measured 
by  the  stadia,  and  its  bearing  taken ;  the  photographer  took  the  bearings  necessary  to 
orient  his  views ;  and  means  for  checking  the  orientation  were  supplied  by  bearings, 
taken  from  the  stadia-stations,  of  prominent  points  that  would  appear  in  the  views. 

On  another  occasion,  M.  Javary  made,  in  six  hours,  a  photographic  survey  of  the 
ground  included  in  a  zone  three  fourths  of  a  mile  in  width,  in  front  of  a  fortified  place  ; 
and,  after  8  hours  expended  in  the  plotting,  presented  the  commanding  officer  of  the 
attacking  force  with  an  exact  and  complete  map,  including  the  levelling ;  and' this  in  rainy 
weather  and  with  wet  plates.  He  states  that,  with  four  operators,  the  exact  survey  of  any 
fortified  place  can  be  made  and  plotted  within  24  hours. 

As  an  instance  of  its  value  in  defensive  operations,  views  of  the  surrounding  country 
may  be  taken  from  the  different  salients  or  other  commanding  points  of  a  work  ;  the  dis- 
tances to  the  enemy's  batteries,  etc.,  ascertained  from  the  plotting,  can  then  be  noted 
on  the  representations  of  the  objects,  and  the  prints  distributed  to  the  chiefs  of  pieces, 
who  are  thus  enabled  in  an  instant  to  get  the  range  of  any  contained  point. 

Aside  from  the  merit  of  this  method  as  a  means  of  military  surveying,  the  rep- 
resentation by^  photographs  of  all  the  details  possesses  a  special  advantage  in  camp  or 
field ;  it  may  happen  that  a  knowledge  of  some  feature  of  ground,  which  in  ordinary 


64  PHOTOGRAPHY  APPLIED    TO   SURVEYING. 

topographical  sketching  might  be  omitted,  would  prove  of  great  value ;  again,  the  natural- 
ness of  a  photographic  representation  might  convey  to  some  the  information  not  afforded 
them  by  a  topographical  map. 

Since  balloon  photography  claims  such  an  important  part  in  warfare ;  the  element  of 
danger  to  the  aeronaut,  from  the  enemy's  fire,  is  of  considerable  moment.  During  the 
siege  of  Paris,  the  Germans  made  use  of  a  large  rifle  so  mounted,  and  furnished  with 
stock  and  butt,  that  it  could  be  fired  from  the  shoulder ;  this  was  transported  by  wagon 
to  points  over  which  the  French  balloons  chanced  to  pass,  and  some  of  the  aeronauts  tes- 
tified to  hearing  the  bullets  whiz  by  them  when  at  altitudes  of  from  800  to  1000 
yards.  In  November,  1870,  one  balloon,  "  La  Daguerre,"  lost  so  much  gas  from  perfora- 
tion by  the  balls  from  the  ordinary  infantry  rifle,  that  it  had  to  descend  and  was  captured 
by  the  Prussians.  From  experiments  subsequently  made  by  the  French  war  department 
it  was  found  that,  at  an  altitude  of  1300  feet,  a  balloon  13  feet  in  diameter  was  unharmed 
by  the  best  sharpshooters  with  a  chassepot  rifle ;  but  from  the  testimony  above  given,  their 
failure  in  attaining  it  must  have  been  due  to  ignorance  of  the  true  form  of  the  trajectory 
when  the  line  of  fire  is  inclined  to  the  horizon.  It  is  found,  theoretically,  that  with  a 
chassepot  rifle,  model  of  1874,  the  extreme  range  in  firing  upward,  at  angles  between  80° 
and  90°  with  the  horizon,  is  nearly  2100  yards,  while  the  effective  range  as  against 
an  ordinary  balloon  is  about  800  yards  ;  for  angles  between  60°  and  70°,  the  effective 
range  is  increased  to  about  950  yards,  and  between  40°  and  50°  to  about  1000  yards. 
(For  an  elaborate  discussion  of  this  subject,  see  "  Tir  Centre  les  Ballons,"  by  Dufaux,  1886.) 

The    "  Jahresberichte "    enumerates   the    following   conditions   to   be    fulfilled,    in    order 
that   balloons   may   be   generally   adaptable    to   military   service : 
I.  To   be    readily  transportable,   either   inflated    or   empty. 

II.  That    the    envelopes   shall    be    resisting   and    impermeable    to    the    contained    gas, 
in    order   that   they   may   lose   as   little   as   possible   of   their   ascensional    force. 

III.  The    means   to    be   supplied    for  filling  them   rapidly,  in    any  place  whatever,  and 
of   replacing   the   gas  lost. 

IV.  For  free  balloons:  power  to  move   in  any  direction,  of  landing  promptly  and  with 
security,   also  of  rising  promptly. 

V.  For  captive  balloons :  means  for  preventing  rotation  about  a  vertical  axis,  and  for 
causing  them  to  rise  in  good  condition  notwithstanding  the  wind. 

Experiments  very  lately  made  in  France  and  England  have  been  successful  as  to  the 
first  three  conditions  ;  as  may  be  observed  in  the  preceding  pages  (Section  VI.),  the  fourth 
and  fifth  are  in  a  fair  way  of  fulfillment ;  and  it  certainly  seems,  after  having  within  a 
comparatively  short  period  surmounted  so  many  obstacles,  that  the  efforts  made  to  perfect 
this  valuable  aid  to  scientific  investigation,  for  civil  as  well  as  military  purposes,  will  soon 
meet  with  a  favorable  termination. 

66.  Some  of  the  Advantages  Derived  from  its  Use. — In  photographic  surveying,  an  en- 
tire tour  of  the  horizon,  or  any  desired  part  thereof,  is  obtained  in  a  few  minutes  by  a 
series  of  very  simple  operations,  involving  no  multiplied  or  fatiguing  observations ;  no 
drawings,  sketches,  or  lines  of  direction  difficult  to  trace  in  the  field  ;  no  delicate  pointings 
or  minute  readings,  and  no  prolonged  computations.  There  is  no  fear  of  having  forgotten, 
or  omitted,  some  important  point ;  because  every  visible  detail  is  represented,  and,  when 


VARIOUS  APPLICATIONS   OF  PHOTOGRAPHIC  SURVEYING.  65 

negatives  are  used  in  plotting,  with  a  precision  that  is  rarely  exceeded  ;  there  is  also  no 
occasion,  as  in  other  methods,  for  rejecting  erroneous  or  doubtful  observations. 

The  office-work  presents  no  difficulties ;  the  constructions  are  simple  and  rational ;  and 
errors  of  destination,  pointing  or  orientation,  are  made  impossible  by  the  numerous  verifi- 
cations which  serve  at  each  step  to  check  results  already  attained.  All  the  work  done  in 
the  field  is  of  value,  and  there  are  no  regrets  for  not  having  made  all  useful  observations 
at  any  station. 

The  map  finished,  the  photographs,  as  addenda,  present  as  already  stated  a  view  as 
perfect  and  comprehensive  as  if  the  observer  had  personally  traversed  and  inspected  the 
tract  or  region  represented. 

The  only  inconvenience  that  seems  to  present  itself,  is  that  of  acquiring  a  certain 
degree  of  skill  in  the  chemical  manipulations;  but  this  part  of  the  work  is  now  reduced 
to  such  simplicity  that  but  little  practice  is  necessary  for  the  purpose ;  besides,  in  surveys 
of  considerable  extent,  a  professional  photographer  would  usually  be  retained. 

Finally,  in  the  prosecution  of  a  survey,  the  stations  may  often  be  so  selected  that  in 
the  views  artistic  effect  and  necessary  data  are  joined,  and  the  resulting  photographs  would 
thus  be  made  as  interesting  in  the  former  respect  as  they  are  useful  in  the  latter. 


INDEX. 


PAR.  PAGE 

33.  Accuracy  Attainable  (see  also  pars,   i,  51,  52,  61, 

62,  and  65), 34 

64.  Addenda  to  Maps,  Use  of  Photographs  as,    ...  62 

40.  Adjustment  of  the  Optic  Centre,  Cylindrograph,    .  38 

32.  Advantage  in  the  Use  of  Negatives, 33 

44.  Advantages  claimed  for  Cylindrograph,     ....  40 
64.                         derived   from   Applications  of  Photo- 
graphic Surveying, 62 

28.  Angle  Scale,  Vertical, 31 

57.  Apparatus  for  Balloon  Photography,  Special,     .     .  55 

APPLICATIONS  OF  PHOTOGRAPHIC  SURVEYING,       .  62 

63.  To  Farm-surveys  and  Civil  Engineering,    ...  62 

64.  To  Geographical  and  Archaeological  Description, 

and  as  Addenda  to  Maps, 62 

65.  To  Military  Purposes, 63 

66.  Some  of  the  Advantages  derived  from,       ...  64 

3.  Azimuth  and  a  Distance,  Location  of  Points  by,     .  5 
39.  Azimuth  and  Slope  Scale,  for  Cylindrograph,     .     .  38 

BALLOON  PHOTOGRAPHY, 52 

55.  Sketch  of  Progress  in,  to  1880, 53 

56.  Recent  Experiments  in, 55 

57.  Special  Apparatus  for, 55 

58.  Notably  Successful  Experiments  in 56 

59.  Photographic  Outfit  for,  and  Method  of  Working,  57 

60.  Use  of  Small  Balloons,  Captive  and  Free,      .     .  58 

61.  Observations  on  Balloon  Phot.  Surveying,      .     .  59 
61.   Balloon  Phot.  Surveying,  Observations  on,        .     .  59 
60.   Balloons,  Small,  Captive  and  Free,  Use  of,        .     .  58 
23.   Best  Time  for  Field-work 24 

Camera,  Early  Forms  of,  see  "  Historical  Sketch." 

7.         "      ,  for  Plane  Perspectives, 9 

36.          "      ,   "    Cylindric        "         , 36 

46.         "      ,  "    Radial             "         , 42 

52-                 i   "                            "         ,  Mangin's,  ...  47 

53.  CAMERA  WITHOUT  A  LENS, 50 

Captive  Balloons,  see  "Balloon  Photography." 

46.  Chevallier's  Camera, 42 

10.  Changing-boxes,       n 

33.  Choice  of  Scale, 34 

63.  Civil  Engineering,  Application  to, 62 

9.  Compass  and  Level,  for  Use  with  Camera,    ...  to 

30.  Contouring  from  Photographs,    .     , 32 

35.  Cylindrograph,  Topographic- 36 

30.  Datum  Plane,  Reduction  of  References  to,    ...  32 

4.  Definitions  of  Terms  used  in  Perspectives,    ...  5 
17.  Development  of  Photographic  Plates,       ....  17 

5.  Direction  of    a  Point,  To  Determine  from  a  Per- 

spective Representation, 5 

13.   Distance,  Focal,  Measurement  of, 13 

16.   Distortion,  Test  of  an  Objective  for 16 

19.   Distribution,  and  Selection,  of  Stations 19 


PAR.  PAGB 

4.   Drawings,  Perspective,  Relation  of  Photographs  to,  5 

63.  Engineering  Purposes,  Application  to 62 

51.  Error,  Instrumental,  of  Cylindrograph 46 

56-58.   Experiments  in  Balloon  Photography,      .     .     .55-6 

17.   "  Exposure"  of  Photographic  Plates, 17 

63.  Farm-surveys,  Application  to, 62 

15.  Field  of  View,  Measurement  of, 16 

FIELD-WORK  : 

24.       General,  Conditions,  Methods, 25 

19.  Selection  and  Distribution  of  Stations,  ....  19 

20.  Method    I. — With  Triangulation  Established,     .  20 
.21.             "        II. —    "     Camera  and  Hand-compass,  .  22 

22.  "      III. —    "     Camera  only, 23 

23.  Best  Time  for, 24 

43.       In  Case  of  Cylindric  Perspectives 39 

47.            "         of  Radial                "          , 44 

47.   Fixed  Sectors,  Operating  by, 44 

17.   "  Fixing"  of  Photographic  Plates, 17 

13.  Focal  Distance,  Measurement  of, 13 

20.  Form  of  Record  Used  in  Field-work  (Method  I.),  .  21 
Free  Balloons,  see  "  Balloon  Photography." 
GENERAL  PRINCIPLES, 4 

2.  Range  and  Object  of  the  Method, 4 

3.  Location  of  Points 4 

4.  Relation  of  Photographs  to  Perspective  Draw- 

ings, Definitions  of  Terms 5 

5.  6.  To  Determine  the  Direction  and  Position  of  a 

Point  from  a  Perspective, 5,7 

64.  Geographical  Description,  Application  to,     ...  62 

52.  Glass  Reflector,  Mangin's, 49 

21.  Hand-compass,  Method  with  Camera  and,    ...  22 

27.  Heights,  or  References,  To  Determine,    ....  28 
29.         "      ,  Mechanical  Measurement  and  Scale  of,     .  31 
50.         "      ,  To  Determine,  in  Radial  Perspectives,     .  46 
43.         "      ,    "           "         ,  in  Cylindric         "        ,     .  39 
61.          "      ,  in  Balloon  Photography, 59 

14.  HH', — the  Horizon,  To  Determine 14 

r.  HISTORICAL  SKETCH, i 

14.  Horizon,  or  HH',  To  Determine, 14 

Lens,  see  "Objective." 

9.  Level,  for  Camera 10 

Levelling,  see  "Heights,  or  References." 

3.  Location  of  Points 4 

54.  LONG  RANGE  PHOTOGRAPHY 52 

52.  Mangin's  Camera, 47 

34.  Map,  Photographic,  Explanation  of, 35 

64.  Maps,  Use  of  Photographs  as  Addenda  to,  .  .  .  62 

15.  Measurement  of  Field  of  View, 16 

13.               "           of  Focal  Distance, 13 

28.  Mechanical  Measurement  of  Angles 29 

29.  "                    "            of  Heights 31 


68 


INDEX. 


PAR.  PAGE 

52.  Metallic  Reflector,  Mangin's .     .  47 

65.   Military  Purposes,  Application  to, 63 

Minor  Surveys,  see  "  Farm-surveys." 

35.  Moessard's  Camera, 36 

48.  Movement,  Continuous,  Operating  by,     ....  44 

10.  Negative-box, n 

32.   Negatives,  Advantage  in  Use  of 33 

17.           "        ,  Development  of, 17 

2.  Object  of  the  Method 4 

37.  Objective,  and  its  Carrier,  Cylindrograph,     ...  37 
8.           "       ,  Description  of,  Kind  Required,    .     .     .  10 

16.  "       ,  Test  for  Distortion 16 

"       ,  For  other   Kinds  of,    see    Text    under 

"  Camera." 

14.   Objective  Slide,  and  Objective-slide  Scale,    ...  14 

61.   Observations  on  Balloon  Photography,     ....  59 

13.   OP — Focal  Distance,  Measurement  of,     ....  13 

48.  Operating  by  Continuous  Movement, 44 

47.           "          "   Fixed  Sectors, 44 

17.  Operation  of  Taking  a  Photograph, 17 

Optic  Axis,  see  "  Tests  and  Measurements." 

13.  Optic  Centre,  To  Find  Position  of, 13 

25.  Orientation  of  Views,  Definition  of, 25 

42.             "                  "     ,  Device  for,  Cylindrograph,  39 

26.  "                   "      ,  Special  Cases  of,    ....  26 
59.  Outfit,  Photographic,  for  Balloon  Photography.     .  57 

7-10."     ,  "  ,  General 9"11 

i.   Panoramic  Apparatus,  Early  Forms  of,    ....  I 
4.   Perspective  Drawings,  Definition  of  Terms,       .     .  6 
i.                               "         ,  Early  Use  of,  in  Map  Con- 
struction,        i 

4.             "                 "         ,  Relation  of  Photographs  to,  5 

PERSPECTIVES,  CYLINDRIC,  METHOD  BY,     ...  9 

"           ,  PLANE,                            ,    ...  36 

,  RADIAL,              "            ,     ...  42 

1 7.   Photographs  for  Surveying  Purposes,  How  to  Make,  1 7 

34.   Photographic  Map  (Plate  I.),  Explanation  of,    .     .  35 

4.   Photographs,  Relation  of,  to  Perspective  Drawings,  5 

25.   Plan,  Construction  of  the, 25 

PLANE  PERSPECTIVES,  METHOD  BY, 7 

45.   Plane-tables,  Photographic, 42 

38.  Plate-holder  for  Cylindrograph, 37 

7.            "        ,  Ordinary, 9 

10.   Plates,  Sensitive,  or  Photographic, n 

PLOTTING: 

24.  General  Considerations, 25 

25.  Construction  of  the  Plan, 25 

26.  Special  Cases  of  Orientation, 26 

27.  To  Determine  Heights  or  References  of  Points,  28 

28.  Mechanical  Measurement  of  Horizontal  and  Ver- 

tical Angles,  Tangent-glass  and  Vertical  Angle 
Scale 29-31 

29.  Mechanical  Measurement   of   Heights,   Scale  of 

Heights, 31 

30.  Reduction  of  References  to  a  Datum  Plane,  The 

Contouring, 32 

31.  Suggestions  in  Regard  to 33 


PLOTTING — ( Continued:) 

32.  Preparation  of  Prints  for,  Advantage  in  Use  of 

Negatives,  Transparencies, 33 

33.  Choice  of  Scale,  Accuracy  Attainable,  ....  34 

34.  Example  of  a  Photographic  Map 35 

43.       From  Cylindric  Perspectives, 40 

49.          "      Radial                            , 46 

61.           "       Balloon  Photographs, 59 

8.   Pneumatic  Shutter,  Reference  to, 10 

5.  Point,  To  Determine  Direction  of,  from   Perspec- 

tives,       5 

6.  "     ,  To  Determine  Position  of,  from  Perspectives,  7 

3.  Points,  Location  of, 4 

27.          "    ,  To  Determine  References  of, 28 

32.   Preparation  of  Prints  for  Plotting,       .....  33 

PRINCIPLES,  GENERAL, 4 

17.   Printing  Frame  and  Printing  from  Negatives,    .     .  17 

32.   Prints,  Preparation  of,  for  Plotting 33 

RADIAL  PERSPECTIVES,  METHOD  BY 42 

2.   Range  of  the  Method, 4 

20.  Record,  Form  of,  for  Field-work 20 

30.         "     ,      "           "    Reduction  of  Heights  to  Com- 
mon Datum, 32 

27.  References  of  Points,  Determination  of,    ....  28 

30.  ,  Reduction  of,  to  Datum  Plane,  32 
52.   Reflectors,  Glass  and  Metallic  (Mangin's),     ...  47 
12.   Register,  To  Test  for 12 

4.  Relation  of  Photographs  to  Perspective  Drawings.  5 
10.   Roller  Plate-holder,  Reference  to, n 

28.  Scale,  Vertical  Angle 31 

29.  "       of  Heights 31 

39.       "    ,  Azimuth-  and  Slope-,  Cylindrograph,       .     .  38 

35.  "    ,  Choice  of, 36 

47.  Sectors,  Fixed,  Operating  by, 44 

19.  Selection  and  Distribution  of  Stations 19 

10.   Sensitive  Plates  and  Changing-boxes, ir 

60.  Small  Balloons,  Captive  and  Free,  Use  of,    .     .     .  58 

66.  Some  of  the  Advantages,  in  Applications  of  Subject,  64 

57.  Special  Apparatus  for  Balloon  Photography,     .     .  55 

26.         "       Cases  of  Orientation, 26 

19.  Stations,  Selection  and  Distribution  of 19 

31.  Suggestions  in  Regard  to  Plotting, 33 

63.  Surveys,  Farm-,  Application  to 62 

28.  Tangent-glass,  or  -Scale, 29 

i.   Telemetrograph,  Reference  to  the i 

TELESCOPIC  PHOTOGRAPHY 52 

4.  Terms  Used  in  Perspective,  Definitions  of,  ...  5 

12.  Test  for  Register, 12 

16.  Tests  and  Measurements  Required 16 

23.  Time  for  Field-work,  The  Best 24 

17.  Toning  of  Photographs 17 

35.   Topographic-Cylindrograph, 36 

32.  Transparencies,  Use  of,  in  Plotting 34 

20.  Triangulation,  Reference  to 20 

28.  Vertical  Angle  Scale 31 

6,  27-8.  Vertical  Angles,  Measurement  of,      .     .     7,  28-31 

28.                         "                          "              ,  Mechanical,  30 


.  o£, 


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GENERAL  LIBRARY  -  U.C.  BERKELEY 


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